#1&#2: One reason is money has already been invested in tooling, so if everyone stops using connector A and starts using connector B, connector A will drop in price (down to cost of materials+labor) and someone will say "Why are we paying twice as much for connector B?" This goes for metric vs imperial, power supplies, rotors, wheels, and so on

#1&#3: If you come up with a new design, you get royalties if others use it and you don't have to pay royalties to someone else.