Simple, really. When the company of Bungie itself split on bad terms with Microsoft they needed a good bit of funding to help with game making, so they sided with Activision and Sony eventually came along and paid them a sport of cash so that they would be able to entice players with exclusive gear and cosmetics to lure players in with the appeal of playing on a PlayStation console. This was both a successful and haphazard strategem seeing as how most of the players familiar with Bungie already had some sort of gaming experience with them, most of whom were players on the Xbox consoles.