I imagine the American networks are waiting until all the TNG seasons are remastered before buying the rerun rights.
The only place TNG is currently playing in the States is on BBC America. Not sure too many channels are falling all over themselves for the HD rights.
This is just personal opinion, but my intuition tells me TOS is the only "keeper" Trek in the pantheon, the only Trek that's truly entered the long term public "zeitgeist" (hate that word, can't believe I just used it).
I feel the other Trek's, even TNG, will steadily decrease in their impact and extended awareness in popular culture - DS9, VOY and ENT are probably already a long way down the hole I'm sad to say.
It's always been about Kirk, Spock and Bones (Uhura)! Probably always will be - I'm not too unhappy about that though!