Choice of teacher (conventional)
Nowadays, there are in fact a few places on Earth where there are lots of mandolin teachers, but these places are still not very common. Let's face it: it's really not very likely that a suitable teacher lives just around the corner. And even if one does, people are sometimes prepared to accept a lot of inconvenience to have lessons with a particular teacher.
Often, of course, it's just impossible to reach any mandolin teacher at all.