Verizon basically won't let unlocked phones onto their network. In order to work on Verizon, the phone's ESN must show up on Verizon's master list. To some degree it is a limitation of CDMA technology, where the ESN was coded into the phone (rather than a SIM, like GSM). Neither Spring nor Verizon have been good about allowing phones on their network unless it is sold by them -- though they have shown some softening of the position with Google (and the Nexus phones).