Because when you pull in the kind of money that this thing did from a couple days of special screenings someone takes notice and wonders what one with a feature length and feature budget might be able to gross. If it is done as a BBCA or BBCW production I don't think it has the same limits on it as if it is a straight BBC one. Didn't the theaters have additional content anyways with the making of thing and the intro?
And something similar is the only kind of added content any future theatrical screened episodes could have. And they would only be able to get screened on one or two days.
If someone wanted to try to cash in with a full feature length and feature budget production, then the only real option open is to fast-track that rumoured David Yates movie, which would presumably be separate enough from the TV show to avoid any murky ground with the license fees.