Both C#m and Db can be played on piano using only the black keys (skipping the 3rd note of the scale). This makes them easy keys for beginners. I'm not sure if that's the reason, but it could be related.
Anecdotally, I know a few vocalists that sound great in these keys and use them as a starting point
As a belated followup, I should observe that if you're playing "in C sharp minor" on the black keys, you're skipping notes 3, 6, and 7 of the scale... and those are the only notes that differ between a minor scale and a major scale, making the "minor" designation completely meaningless.
From the Netherlands, UK Office, as that one was broadcast on Dutch television already before the American version existed. So I guess the answer depends on your age :)