There was actually a technical reason for the restriction on porting within the same rate center. On interLATA or intraLATA toll calls, the IXC is responsible for performing the LNP query and identifying the terminating LEC; the originating LEC is responsible for identifying the call as an IXC-handled call and handing it off to the proper IXC. On local calls, the originating LEC is responsible for performing the LNP query and identifying the LEC of the destination number. With the restriction that numbers can only be ported within the rate center, the originating LEC can do the local-toll differentiation based on the dialed number. Without that restriction, the originating LEC would have to do an LNP query on every call, because the Location Routing Number would be needed to do the local-toll differentiation.

That additional database and signaling network query load is not trivial. In addition, the restriction allowed LNP to be transitioned in by rate center - as a rate center was opened for portability, the class 5 switches in that rate center began doing queries on local calls, and the IXCs began doing queries on toll calls into that rate center. Without that restriction, LNP would have had to be flash cut - every class 5 switch in the NANP would have had to start LNP queries on every call as soon as any number could be ported, because class 5 switches don't have knowledge of what NPA-NXXs are in what distant rate centers.