I can imagine the original reason was gear ranking. They don't want us getting a new item, going into private, and boosting it up. Same goes for general XP, stats, etc. I mean, Bungie can't separate pvp and pve weapon stats, how could they possibly separate private vs public? Now that we can use motes, I can only guess the stats as said above is the main reason. That, and the introduction of private will devastate Crucible numbers. And we all know Bungie favors their precious Crucible over everything.