though it may also have to do with the fact that certain stories can't be told without the space--and time--of a longer piece, and thus get written less often

Or the mental effort? Making up Auditorial cases worthy of Miles is hard. It's always the plot that stymies me, and that I try to handwave or avoid. Whereas I can do emotional complication till the cows come home (and moo despondently and feudally).

But yes. I suspect there are lots of reasons that the one-shots are one-shots, but it's very interesting that they exist and what they turn out to be.