A number of reasons probably- people had bad experience with them before so avoid them, some don't like technology at all (old or young), some 'think' that it's just faster to go to the cashier line than the self serve (not really if you do everything right). And.. some do it out of habit- it's something they're used to doing.
Some of the self serves CAN be quite picky especially with the baggage and scales areas even if you do it right- I've had that happen to me more times than I can count. Have also done some dumb stuff and needed a cashier's help to reset it lol. Doesn't stop me from using it... as I don't like interacting/being social most of the time (I'm still nice but in general the introvert in me comes out).
