Because destiny was held back be the by the previous gens. I remember back when we were all complaining about vault space at the beginning and bungee said the one systems memory or whatever couldn't handle showing that many items on screen. This is only one example and destiny 2 will be larger and even more difficult for it to handle. Have you seen a comparison between bo3 for the ps4 vs the PS3 it's PATHETIC. Destiny probably could have looked a lot nicer and ran a lot smoother but they had to make sure it could run on the old gem systems as well.