It's easier to provide earlier upgrades for the iPhone, as their return on investment is higher than other smartphones. Still costs AT&T an arm and a leg, but with the improvement in data services over the last year, (and hopefully with a newly designed iPhone 4 with better antennas) better voice services also.
I still believe that the iPhone won't make it's way onto Verizon for a while unless AT&T doesn't deliver on it's promise to Apple to improve their network even further.
Note, AT&T spend $9 billion on network upgrades in 2009 (Verizon spend $7 billion). AT&T intend to spend $18-19 billion in 2010.
Verizon are in a good spot with their 3G, but the transition to 4G is going to be rough. 4G LTE is not backwards compatible with their 3G CDMA. That means when they roll out their 4G network, they will need 2 chips for each network. It's still unknown how they will be able to swap from chip to chip seamlessly.
AT&T (and T-Mobile) have a 3G network that is backwards compatible, so they will only need the single 4G chip and it will work fine on the 3G GSM network.
Since Apple isn't really a "phone company" (like RIM and Nokia), they may not want to invest heavily in developing a dual phone that has 3G CDMA and 4G LTE chips and having to make them work together. They have had many documented problems making their current setup to work better with AT&T's network (until AT&T had to go and help them with it).
I'd love to see 10 million iPhones on the Verizon network... as it would also go down in flames and it would shut up the Verizon fanboys.
