Microsoft needs Verizon (and other carriers) more than they need Microsoft. So Microsoft is at a disadvantage and has to cater to carrier demands (exclusive agreements).
It also doesn't help that the relationship between Microsoft and Verizon was damaged because of the Kin.
Look it up, you know who killed the Kin ? Verizon did, having a phone geared towards younger people but, required a $30 a month data plan to use the phone and that is what killed it. The costs to use it was just too much for what you got on it.
I bet if you re-released the Kin today, just updated for modern social networks (and their features) with a little sleeker/modern design and put a $15 a month 2gb plan on it, it would sell like hotcakes.
Something for the high school kids who just want texting and facebook...