numbers... pure and simple. How many dumbass hicks do you see when you go in public wearing a jeff gordon t-shirt or some other guy who gets paid too much to drive around in a circle? TONS!
Now ask yourself how often you see someone with motoGP apparel? never.
Now ask yourself what the better portion of sportbike riders do with thier bikes... (answer: ride around town and hit up the occassional twisty backroad... yes, I'm very guilty of this one)
Therein, you have your answer. There's millions of people who like nascar, so millions will watch, and ratings will be high. But the GP scene in the US is very very lacking. Over in europe I'm sure it's played more cause it has a bigger following. Here... it's more like a group of enthusiasts.
To answer your original question though... the races are usually on speed channel on sundays.