The problem is that postrock has thrown out the baby with the bathwater. There's nowhere new musicians can take the stuff, and as much as it appeals to listeners reared on college radio stations and "indie music," you can't help but suspect that you only like it because you yourself are growing old. Rock, for all its faults, gets the pulse going. Postrock, after a while, just seems like innovative easy listening, as the piles of unbought CDs in the record stores' relevant sections attest.
(I myself don't think that the ideas behind postrock will die, even ig postrock itself doesn't outlive the early noughties. Take, for example, early-90s shoegazer; it was swept into commercial oblivion by grunge, but yet musicians kept making low-key, introspective music, albeit with stylistic variation. And rock'n'roll, that old post-war baby-boomer staple, is itself looking rather tired and lacklustre.)