Sprint and Verizon are using CDMA where AT&T use GSM systems and they are not compatible, this would be the biggest hurdle to unlocking unless every phone produced is made Quad Band or higher.
And why can't they? It's not like that's a hard request...nor is it as if we don't have the technology to do so. The service providers seem to be the ones trying desperately to hold on to their exclusivity. Because God forbid we provide them with a little competition - which could lead to better business practices, and better service & savings to us as consumers.
It's pretty much the same with cable companies, too...in a lot of places, Comcast, etc. basically have a monopoly of a certain area. So where's their incentive to provide better service and savings to their customers? They can charge whatever they want, and limit their customers however they want, and we can't say shit because there's no one else to turn to. The cell phone companies play ball in almost the same way.