I'm not sure why Bullock would run for the House, given he'd have had a much better platform to run for President in 2024/2028 as a Senator...
I never thought he would run for the House this year, but if I had to give a reason for him to do so, the top reason would be that the House was an open seat this year, unlike the Senate which had an incumbent running for re-election, and thus he might have started out as a favorite for this open House seat (and thus increase his chances of actually being elected to Washington in some capacity).
However, under current circumstances the best chance for him to actually be elected to Washington would be if MT regains its second house seat in 2022 and he decides to run for the western seat then.