I assumed he did the bumpstock ban in order to have something to point to during the re-election debates regarding gun violence, which was a guaranteed debate topic that makes Republicans look bad (because 'dead children'). But then Covid came along and there weren't any major shootings to cause gun violence to come up in the debates.
Plus, I think he knew the courts would throw it out anyway, down the road. So in the long run, no lasting harm and yet the bonus of 'See, I did something to stop gun violence!' for re-election.

Just my theory.