I think he's just lost his passion. Hell, I liked Ghost of Mars, but even I could say he was phoning it in, sorta. Not to mention that three of his last four movies were remakes, two of which were remakes of his own earlier films (Escape from LA is practically a mad libs version of Escape from NY, Ghosts of Mars to Assault on Precinct 13, less so).
I don't think he's ever given a crap about his critical reputation, I think he hasn't made anything significant in years is because the times have passed him by, so to speak. Even during the 90's he was making the same kinda films he made in the 80's, more or less the exact same way. I'm sure he could get a film going if he wanted to, but I don't believe he has the patience to put up with any studio meddling, or that he's capable/willing to change his methods. And I doubt seriously that he's chomping at the bit to make a neutered, PG-13 version of a John Carpenter movie in 2009 to appease the kids who weren't even born when he was in his heyday.
What is Carpenter up to anyway?
Having checks placed in his hands by production companies doing remakes of his masterpieces.
He said he signed up to the Halloween remake for purposes of quality check. He signed the contract, they gave him a quality check.
BTW, I understand he's a big videogamer now, loves his Xbox I hear. So if there's an old-sounding guy who's waaaaaaaaaaaay more awesome then you dealing ownage at Halo, now you know why.