They capped the factions at 390 because you could artificially bump your level by spamming (heavy ammo, motes, planetary materials etc) and not playing bungie's way...even though they put all those things in the game. It bets me as to why regular crucible drops can go to 400... But this game now seems heavily weighted towards pvp. As for pve... You can only raid, get exotic engrams, nightfall or strike chests to get over 390.