Do you use the Direct Impingement or Gas Piston system?
Reason 1. There's no real across the board agreement on what GP system is best, so there's no interchangeability. If you need parts, you have to go to the mfg that made your GP upper and hope that, years down the line, they still make those parts.
Reason 2. I don't like GP ARs. I got nothing against GP, just not on an AR.