Two things I can think of is gasoline powered motorcycles aren't that much more efficient than a Prius. And a Prius seats four. I suspect the same applies when you switch to electric[1].

The other one is the injuries per mile are much higher for motorcycles. One aught to account for that as well.

Side issue: Motorcycles are terrible from an emissions standpoint. Though of course an e-motorcycle would be better.

[1] Scaling factor vs size is highly positive.