I think part of the problem is that there are a growing number of old shows (in all genres) that just aren't seeing the light of day - anywhere. Personally, I think the best TV shows of all time occurred in the '60s and '70s.
This will probably get fixed as the literally thousands-of-channels universe dawns. Instead of the relatively few nostalgia channels now available, there could be many dozens. So really obscure (by anybody's definition) old shows may get their day in the sun.
Still, the current situation isn't completely grim for the oldies. I checked the upcoming seven-day schedules on the nostalgia specialists and some other outlets. Came up with a pretty impressive list of fifties/sixties/seventies shows...
TV Land: All in the Family, Sanford & Son, Three's Company, Brady Bunch, Happy Days, I Dream of Jeannie, Bewitched, Gunsmoke, Bonanza, Mister Ed, Green Acres, I Love Lucy, Dick Van Dyke Show, Munsters, Leave it to Beaver, Andy Griffith Show
Good Life: Combat, FBI, Honeymooners, Mayberry RFD, New Dick Van Dyke Show, 77 Sunset Strip, Hawaiian Eye, Surfside 6, Gallant Men, Maverick, Bronco, Cheyenne, Superman, Lawman, F Troop, Sugarfoot
GSN:What's My Line, Number Please, Match Game, Dawson Feud, Perry Card Sharks, Beat the Clock
SciFi: Twilight Zone, Star Trek, Kolchak, Battlestar Galactica, Outer Limits
Hallmark: Hogan's Heroes, Beverly Hillbillies, Waltons, Gilligan's Island, Perry Mason, Rawhide (some duplicates eliminated)
ESPN Classic: many games from as far back as 1954
This is hardly an exhaustive list, of course. I just browsed through the zap2it schedules for a few channels. Other cable networks and local channels also carry things like M*A*S*H, Hawaii Five-O, Lawrence Welk(!). Maybe these shows aren't quite obscure enough, though some are pretty out-of-the-way for me. But I was kinda surprised at just how much regularly scheduled 50s/60s/70s TV is available already on a number of outlets.
A lot of old television has just been lost. Things start getting spotty before the mid-seventies in many genres, especially for live and videotaped shows. Johnny Carson's first Tonight show sleeps with the fishes, for instance.
But you could easily spend every waking hour watching nothing but currently scheduled 50s/60s/70s shows. And as I said, the number of old shows getting a crack at the schedules will probably rise over the new few decades as the TV universe starts to look more like today's Internet.