Sharp-keys are in some ways nicer to play than flat ones.
The preference for these keys is perhaps most extreme in folk styles: going through two collections of celtic-style fiddle music, I found the distribution to be this crass:
- ♭♭ : 5
- ♭ : 5
- ♮ : 7
- ♯ : 57
- ♯♯ : 70
- ♯♯♯: 9
In particular D major is really a great favourite. In fiddle playing, this is also easy to explain: you want to use empty strings much more than in classical music. With more than two ♭s that's simply impossible on the high violin strings, but even for B♭, F, and C major the empty strings occur mostly on thirds. That is problematic, because these notes are the ones you need to do most intonation-adjustment on (upwards for leading notes, down for chords/double stops). In D major OTOH you have the fundamentals of the primary chords on the empty strings, which is perfect for droning etc.; while the thirds are all fingered and can be moved as necessary.
Now, empty strings are somewhat shunned in much of classical music, but in particular for solo playing (think the Bach Cello Suites!) they also have quite a significance. These are actually something of a counterexample though, with both Suite IV and V featuring 3 ♭s. However, since it's totally solo, you can much better "cheat": instead of adjusting the thirds, you adjust everything around them...
Otherwise, it's certainly a bit of a self-fulfilling truth: because the simple tunes are mostly in those keys, those are best practised, hence even advanced players may be a bit more firm in them than in ♭-heavy keys.