Put simply, films like Blazing Saddles made fun of racism without necessarily making fun of racists. They showed the institution as silly and absurd without having to look down and mock the people beyond an initial "Well, yeah, they're bumpkins". A modern film would likely lampoon them mercilessly and dehumanize them.
Additionally, the comic talent those films drew from comes from a very different lineage. Folks like Gene Wilder and Richard Pryor and Mel Brooks have a very different frame of reference for how to do comedy, how to pace it, how to deliver it. I find that modern comedians, having come up with the increasing HBOfication and Comedy-Central-style shows, tend towards very similar schticks: generally progressive-leaning (though the non-progressive stuff is awful redneck pandering as well!) inclusionistic near nihilist little screeds on stage. These tend not to lend themselves to being cast as anything other than a reflection of their own brand in a movie.
I won't really make the "Movies are all PC" argument, because that's silly and overused, but I will say that from a pure marketing standpoint it's difficult to sell a movie as thoroughly dirty as say Sleeper or cleverly offensive as Blazing Saddles: audiences are not interested in that sort of comedy anymore in a world of Soul Plane and Larry the Cable Guy and so forth.
