What's strange is: The conductor always open the door. Sometimes passenger have to wait quite a long time till the door open.
People in Switzerland activate the emergency button if the train doesn't open the door just after its arrival.
Why cannot the driver open the door??
It depends on the route. Most trains in the UK operate with a driver and commercial guard, who is expected to open and close the doors and check/sell tickets. Outside of the South East of England, the vast majority of routes are ran like this (there are a few exceptions e.g. Glasgow). The driver may physically be able to open the doors, but they are not permitted to do so.
In some areas, trains can operate without guards and they are called DOO routes, Driver Only Operation, the driver opens/closes doors. There is no guard, but there may be a ticket inspector.
There are also a few companies where the driver is allowed to open the doors but a guard must close them. Virgin Trains is a prime example.
People are used to waiting for doors on most routes. The guard is usually busy, and then has to travel to the rear cab (or any door on more modern units), to open the doors.
Guards opening doors is a very sensitive issue in the UK.