When writing novels, why would authors spell out timestamps, like writing “five-fifteen” or “quarter past five,” rather than simply “5:15”?

My publisher requires numbers to be written out if they’re under 100; that’s standard practice in prose in general, or was when I learned to write. I was able on one occasion to argue that the military convention in the UK in the 18th and early 19th centuries usually wrote the numbered regiment as “9th (or whatever) Regiment of Foot”, or Dragoons, or Cavalry, etc.

Other numbers are written out.