The engine for Destiny was built well before the current gen consoles were released or even thought of. Since it was built on that platform there was no reason that they shouldn't release the game on that console. Now that current gen are...well...current, dev's know what the capabilities and limitations are of the new machines and can then update their software to work with the hardware. It's safe to assume that the sequel to this game will be a xbone/PS4 exclusive and will not be supported on old gen models, just like it will be safe to assume that all subsequent sequels will be released for whatever they were designed on at the time. I anticipate Destiny 4 and 5 being released exclusively on the PS5 and whatever MS decides to call their next platform.