Let's get a couple of disclaimers out of the way here: women are not objects, they are woefully underrepresented in TV and cinema, and we all really need to seriously examine the way that they are portrayed and the opportunities available to them in modern media. Women are human beings deserving of our respect and admiration, and to think otherwise is simply wrong-headed.
Now, let's talk about titties.
Specifically, let's talk about a notable lack of titties in our horror movies today. This used to be a dependable staple of the genre, which can actually be traced (as so many nudity based things can) to Europe. The 1911 Italian adaptation of Dante's Inferno was one of the first movies to meld horrific imagery with nudity, though it took until the late 20's/early 30's for the trend to hit the US film scene. Cecil B Demil's Four Frightened People was arguably the first of the genre (containing only brief glimpses), and one of the last for quite a while, as Hollywood implemented “The Hays Code” to police morality in film in 1930. This code was repeatedly challenged in the 50's and 60's and was finally abolished to pave the way for our current morality arbiters, the MPAA. Until the voluntary rating system was implemented in the late 60's, nudity was relegated largely to the realm of the porno and the foreign flick. But once it really became an option, filmmakers were free to sprinkle bits of... bits in to their films as they so pleased. All they had to do was avoid that distribution killing “X” rating and they were in the clear.
Plenty of classic horror movies contain scenes of arguably-to-unquestionably gratuitous nude scenes. The one that springs immediately to mind (ignoring my research for a moment) is 1980's Friday the 13th. The only standing rule in this film was that the second your stuff was out, you had sealed your death warrant. This formula held true for every installment of the series: tits = death for the lusty heroine. Shame on the slut, for she displayed her maidenly treasures to the lecherous men around her, so she has to die.
This isn't the only angle the genre took. The 80's and early 90's were replete with films that had seductive femme fatales that used their mammaries to entrance men into their ultimate demise. Angel Heart, The Hunger, Species, Cemetery Man... the list goes on. The movies are arguably exploitative, the nudity arguably gratuitous, but it was there nonetheless.
Until all of a sudden it wasn't. I looked up just a ton of articles, lists, and videos that ran through the “best horror movie nude scenes”, and with the exception of remakes of the classics (Friday the 13th 2009 came up a lot, as did My Bloody Valentine 3D) and homages like the incredible The Cabin in the Woods, almost all of the movies were from the late 70's through the early 90's. Something happened after about 1995 that brought nudity in horror from a standard flow to a slight trickle.
I have a two pronged explanation based on almost nothing but pure speculation and my own experience to account for this (surprisingly, no one has really studied this in an official capacity, at least in terms of what I could find). The first thing I blame is technology. So let me tell you a story about when I was a teenager. I started going to heavy metal shows when I was 15, and something that was almost guaranteed to happen in the lull between bands was a girl getting up on some guy's shoulders, having the audience chant at her, and her showing the crowd her goods. By the time I graduated high school, however, this trend had completely vanished. Like, seemingly overnight. So what happened in the early 2000's to bring about this change? Suddenly, every cell phone had a camera. Girls were no longer just getting a rush out of showing off their milk-makers for a crowd of slobbering doofuses, they were now actively risking having that recorded and indelibly put on the internet, where they could no longer control the exposure. Technology killed tits at metal shows, and it killed tits in horror in a similar vein.
So I'll ask again: what happened in the 1990's that could have had this effect? America Online (AOL, if you're nasty) absolutely fucking exploded. Those discs that everyone had so many of that they could have thrown out all of their coasters were getting installed in record numbers. Between 1995 and 1998, AOL's user base grew by 10 million users (1 in 20 Americans were using the service by the end of that span). This meant that an even greater number of people (as AOL was not the only ISP of the time, just the largest) suddenly had access to just all of the porn. Breasts were no longer a novelty, but an undeniable staple of the life of millions of bap-thirsty Americans. They were no longer wowed by boobies on the big screen, because their small screens had infinity times more, albeit at dial-up speed. Given that movies, and horror movies in particular, live and die off of their inherent novelty, filmmakers had to find new ways to ensure that they could guarantee people would be buying these movies and make up those back-end projections, since the promise of fun-balloons was no longer enough for a large percentage of the customer base.
Reason two goes a little bit deeper than the above, and it requires a little bit of thought as to what drives us, and specifically, what scares us. When George Romero made Night of the Living Dead, it played on our fear of communists. A giant, unthinking horde working towards a singular goal, with no regard for the individual was one of the scariest things the average American could think of at the time. As we moved in to the 80's, where we started to realize that communism was not really the thing we needed to be afraid of, as we were watching it crumble before our eyes, Romero tweaked his formula in the opposite direction. Dawn of the Dead, as most people well know, is about the rampant consumerism spawned in the wake of the red scare. Mindless hordes swarm our center of commercialism as the heroes vainly tried to take shelter in the trappings of material goods and faux security. Skipping past Day of the Dead (where Romero was so ham-stringed by a tiny budget and faithless studio that he had to cut just about any meaning from the final product) we arrive at Land of the Dead, where the world is overtaken by the aforementioned mindless horde, but it is our complete focus on security and willful ignorance that is truly our demise. Obviously, other movies follow a similar pattern, but the <blank> of the Dead series is the cleanest set up here. Our horror movies mirror the fears of the time from which they came.
See, I mentioned above that the voluntary rating system paved the way for filmmakers to include nudity, but that was one of the fastest ways to get your film that dreaded “X” (NC-17 in the parlance of our day) and ensure that no theaters would pick it up for distribution. In the eyes of the MPAA and the moral conservatives, there was nothing more horrifying, more corrupting, than the possibility of youthful eyes seeing exposed bosoms, and that is what the horror champions of the day delivered on. Their approach had to be careful, so as to get just enough boobage to freak out your mom, but not enough that no theater but the seediest porno joint would pick it up. But plenty of directors accomplished this and got more scares than the simple premise of “monster jumps out and yells 'a-bloogy-woogy-woo'”. They could scare the people that weren't even seeing their movies, solely on the promise of their “perverting” methods. Talk about your meta-scares.
But flash forward to today and balloon that statistic I had about AOL in the 90's by “basically everybody”, and we are no longer in an era that fears nudity. The MPAA still has their ridiculous hangups, to be sure. Women are seldom allowed to climax on screen, and a vagina is still an almost blanket “wait-for-the-unrated-version-on-DVD” sticker, but gazongas? Those are relatively benign at this point. Hell, even PG-13 movies get to show bare flesh bongos as long as it isn't for too long in duration (if you want to get really enraged about this brand of idiotic censorship, watch the excellent documentary, This Film is Not Yet Rated). The vast majority of society no longer fears bare breasts, and the horror scene has reacted accordingly.
And now that I have officially used every single euphemism I can muster for “breasts”, I'm going to take a long nap. That shit is exhausting.