Well, T-Mobile also has the Nexus One. That is Google's bread and butter. I mean, my friend with it already has Froyo.
I think it is market share, coverage, and features (of the carriers) mostly. For example who has the most subscribers and largest 3G network? VZW. Who is offering 4G and can roam on VZW's network? Sprint. Who is offering 3.5G that runs as fast as 4G, has a smaller market share, and not as much 3G coverage? T-Mobile.
Google is very smart with putting out superphones on VZW and Sprint. But still, T-Mobile has the Nexus One (which is a superphone).
Also, the Motorola Droid is not running at 800 MHz unless you overclock it. In addition, VZW is getting the LG Ally which has a 600 MHz processor. Not everyone needs a superphone. I think having more middling options is better for the casual users.