Basically, from the perspective of the studio that made it, they have two options:
1) Release it in theatres and make as much money as they can from it, and all the people involved in the movie will thank them, even if it does poorly, because at least theoretically they can make residuals, use it in their CVs usefully, get exposure from it and so on.
2) Can it entirely and don't release it on TV either, and use it as a tax write-off. The trouble is, as I understand it, the more the movie cost, the less effective it is to write it off entirely, and also, you ensure absolutely everyone involved with the movie will naughty word hate the people who made that decision and be harder to work with in future.
It's possible they could have also tried just releasing it straight to paid streaming or something, but that is going to make them even less money than releasing to theatres.
So they went with the overall least-destructive option, which is to release it to cinemas. I think it's suffering more from incredibly terrible reviews than it being part of a "cancelled cinematic universe" - I don't think most movie-goers care about the latter. I do think they care about a 35% RT rating though.