[jcparkyn]

I would guess that it's to do with the fact that musicians generally use the key with fewer sharps/flats. E.g., Bb major (2 flats) instead of A# major (5 sharps). This also follows what's on the circle of fifths, where major keys clockwise from C use sharps, and vice versa.

In the case of drop Db, I would also guess that people just think of it as "slightly lower than drop D", hence just adding a flat symbol.

[tzs]

Nit pick: A# major would be 10 sharps, at least written following the usual convention of each note letter being used exactly once: A# B# C## D# E# F## G##.

[boston_clone]

What is 2 but 1 plus 1? The key of B flat is easier to read.

Disregarding how it'd be read, I suppose you could theoretically get into keys like G-triple-sharp or F-quadruple-flat -- but that's just overcomplicating things, isn't it?