For what reason? You can already play it on next gen consoles and it runs just fine. Running on last gen consoles is an OPTION for those who can't afford or couldn't get there hands on new hardware. Yes, sucks that it doesn't run as well on the old boxes but there are enough people who don't care about the performance and rather play it now than later and removing OPTIONS is always a bad thing.
Because these games are reviewed and advertised as PS4 and X1 games, but cannot run properly on those consoles. Options are always a good thing, but releasing a broken product often hurts the image of the game more than just doing away with that option altogether.