It could be a similar issue as with electricians and electrical contacts: in principle there are far better alternatives to slotted, but because literally every electrician has a number of slotted screwdrivers anyway, slotted is cheap and changing the screw type for a product like a circuit breaker is just a huge hassle for everyone involved nobody bothers.

Edit: actually for circuit breakers the combo of slotted and pozidrive is becomming more common recently. i guess that doesn't work for small screws though.