For me, it's lab costs and ease of shooting (sometimes). As of right now, it's only a couple of dollars more to move up to 220 processing. Why not shoot 220 is my question.
Though i would like and use 220 B&W film, i know a couple of answers to that question.
First, 220 film is often too long. For many occassions, 120 film already is too long.
And even when a large number of frames is exposed 'in one go', changing film isn't that much of a problem. A loaded spare back will do when it's not convenient to reload, and loading when it is convenient is quick enough.
Second, it already is more expensive than 120 film over here.
The film costs more per roll than 2x what a roll of 120 costs. And processing costs double.
So overall, when longer rolls would be fine (on trips), i pay extra for the convenience.
I still would like some T-Max 100 in 220 though. (And hope that they keep producing Portra in 220.)