It seems like only Apple has enough money-making power for carriers to tell the carrier to go screw themselves.
It's pretty simple - Apple owns the hardware and software they're pushing updates for. Sure, they don't own the carriers, but that's not the problem.
In Google's case they neither own the hardware nor all the software because each third party phone manufacturer slaps in its own stuff and maintains its own fork of android.
Microsoft might own the software, but they don't fully own the hardware. Microsoft is a lot closer to managing it than Google though.
This isn't that Apple has more power over carriers and it isn't hard to understand - it's simply that they have more ownership of all the devices and there are far fewer types of devices (very limited set of software/hardware combinations).
Which is a problem because these forks take substantial time to update to the newest version of each OS. And then they take more time because sometimes vendors make modifications to the core OS that breaks Google or other core apps (see e.g. EVO and Gmail) so the whole system has to be QA'd rigorously to prove that nothing is broken.
Apple didn't always offer the stream the updates over the air, and they've always been good about getting the new OS pushed out. I think another part of it is that iPhone users are more likely (or at least were) to plug their phones into their computers, use iTunes, and be prompted to update their phones. That part didn't deal with the carriers at all.