I was thinking about it a little bit and I suppose if you were to compare TV now to TV when I was growing up 30 years ago some things have obviously changed.
One thing is that TV seems more homogenous, if you want to watch cartoons you watch Cartoon Network or Nick, you watch the History Channel and if you're lucky PBS for historical shows, you watch AMC or TCM for classic movies, TV Land for old TV shows and so on. When I was growing up there were only a handful of channels so they showed a more diverse lineup.
The other thing is there is so much more content now. Why would anyone need to watch TOS, when there's TNG, DS9, VOY and ENT with a more modern bent. There's been 40 years of shows more modern than Trek to compete for airtime, not to mention all the alternative media. Again when I was growing up there'd be Our Gang/Little Rascals, Three Stooges, Bugs Bunny, I Love Lucy, King Kong, Godzilla etc etc being shown with all the other programming. There was just more exposure to stuff before our time.
I think the two of these together may explain why with the new TV landscape just aren't as tolerant of older stuff.