If has to do with the network. It requires extra effort from the phone to make up for the network.
Has nothing to do with the network at all... On AT&T which uses gsm has extra channels to do voice and data separate but at the same time. Verizon and Sprint use CDMA which don't have the capability for voice and data at the same time. When Verizon rolled out LTE, and sprint WiMAX and now LTE it gave the option of doing voice over CDMA 1x and data over the 4g network. The tech is there for apple to do so has been for years now. The only fix for this is sprint and Verizon to roll out voice over LTE which may...or may not be compatible. That being said it's the phone not the network.
I'll still be getting the iPhone 5, I like the ecosystem a lot more. But the new lumina's are tempting, if Verizon gets it I dunno