Part of the problem is a lot of players don't know it exists or they think everyone has their own. This thought pops in even before they unlock PVP. If it has then they assume that it's separate because that's how it works in PVp. Or they feel as though they'd use it wrong or waste it. It's how when you do run into the few times you have to use a tank LB you end up dead cause the tank doesn't have it set to their hotbars.