Releasing a game on 4 platforms at once is pretty daunting, a 5th would just be crazy. Not only that but games designed for consoles always get a lot of hate from PC players for being "oversimplified" and not designed for the PC master race. You're also looking at even more servers for PC players which is more initial expenses. I read somewhere about them not wanting to make mouse & keyboard an option for Destiny like some other shooters, which makes a PC release less likely in my mind.