My guess: The MobileMe launch fiasco caught Apple by surprise.
Push Notification will all run through Apple's servers. If they're not ready, they'll get a bunch of bad press all over again for not being able to handle it.
I strongly suspect that 2.1 is perfectly capable of running the push service, but Apple got spooked by their recent server problems and decided to hold off on push notification until they can be sure they can handle the load. So it may be ready, just turned off.
There's even a chance that 2.1 already has it built in, but without the servers being on, it's useless. They may just turn them on one day and say "ok developers, go for it!" You wouldn't even need to update the phone's software. If you have 2.1, it'll suddenly start working. (Or maybe not...it may need 2.2. Dunno.)
All speculation on my part, but given the trouble they had with MobileMe, I wouldn't be surprised if it's a similar problem and they're simply waiting until it's fixed to launch it.