My guess is that very simply none of the VOIP providers have the systems installed or capability to perform number portability, so it has to be done in concert with an ILEC or CLEC which does have the system and capability in place.
ILECs and CLECs put systems in place to be able to perform number porting, and the FCC allowed them to recover their costs of doing so via the Local Number Portability surcharge (which I believe is also being used to produce some additional revenue, but that's another discussion). To my knowledge, none of the strictly VOIP providers have these systems in place or plan to do so in the reasonable future.
This brings-up a question for me regarding the efforts to have VOIP providers be able to receive telephone numbers directly from the NANPA, rather than having to get them from an ILEC or CLEC. I'm presuming this would simply complicate the porting process, but then again maybe it's just a matter of the procedure for porting numbers from a VOIP provider requiring more steps in the process..
Randy Hayes University of Northern Iowa