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

i believe the most popular reason is capo on 1st fret when writing songs, other factors coming 2nd or 3rd (electronic music, sped up old samples, etc)

Electronic dance music is the biggest genre in the data. So then easy to play shouldn't matter. It's still an interesting question. I think playing Db is pretty nice on the piano even if it's not the easiest.

Difficult to play in what instrument?