When banning someone like donald trump you need to have a great press release for justification. You need to point to something that people who are in the middle will read and think, yeah that makes sense, I'm surprised you didn't do that earlier.
Instead, they have this PR that I honestly disagree with, even in the current climate, I don't see it.
I can only think, they wanted to ban him before he left office in order to set a precedent that they could ban him before he left office, and use that for some sort of future legal justification. I could see this as a use to ban future presidents before the 4 years are over. Or if he became president again they wouldn't have to re-instate his account.