My guess, which is only a guess, is the same as the Enterprise's number, back in the 1960s. It looked good on screen, and was easily readable. They may have also been looking for a number that was higher than the Enterprise's for some reason. But, again, that's entirely a guess.
I think that I've read that was the main reason: to indicate that the Reliant
was newer than the Enterprise
and could be a threat to the hero ship in Khan's hands.