Since 9/11 it has always been the rule in the US for two reasons. The first, and most important, is so that the pilot flying doesn't have to get out of his seat in order to open the door. The second is if the pilot flying becomes incapacitated there is someone there immediately aware of it and can also open the door. It has been said that during incapacitation the door can be opened electronically via a code. However the electric lock can be MEL'ed in which case the door can be manually lock which goes back to reason number one.