b keys are more mellow in sound then #. Maybe not so noticeable with a equal tempered tuned keyboard instrument(s) but really noticeable when involving instruments that's not tempered tuned, strings for example. Ab minor to my ear is amongst the most mellow chords there is, but G#minor is not. So this is the reason. When a composer, who is trained to write for an orchestra, writes for for just a piano, I think that the sound of the key and how it would sound orchestrated is so deeply cemented in the head of the composer that this dictates the key chosen.. hence, you probably found that something slow and in minor is more likely to be in a b key where as something up tempo is more likely to be in #.
Of course, then there is also the element of pure delight of being contrary for fun: write something in F#minor and insist that the violinists needs to keep things more flat in pitch because they are playing too sharp on all A, F# and C#... (had you been writing in Gb, that would not have been the case..) Of course, you might get a bow wound around your head or if lucky, just the "look" of the 1st violinist when they realize what is going on and they probably think you're fresh out of a composing class where you didn't do too well on the theory side.
Another really fun exercise is to have a song with a string arrangement using the note F# together in unison with electric bass and guitar.. that one is a real interesting one and the look on the string players trying to match the pitch is great! Definitely NOT the hand position they are used to go to when playing with other "normal" orchestrated instruments for that note.
