I can imagine all sorts of reasons, but who knows?
Maybe the art gets lost or destroyed before it can get to the post office, and the artist doesn't want to admit it
Maybe the artist has a hard time getting to the post office during regular hours and is out of stamps
Maybe the artist has a super-avoidant personality and once they get behind enough to feel guilty, prefer to forget about the problem rather than deal with it.
IDK! None of these are reasonable excuses, of course, but these are the scenarios I can imagine.