This is just wild speculation, but here's my theory: Prior to 9/11, it may have been standard practice for the cockpit door to remain ajar, or at least unlocked, when one of the pilots was absent. After all, if the remaining pilot were to become incapacitated, you'd certainly want the other pilot to be able to get back in, wouldn't you?

Post-9/11, I suspect that they must keep the door closed and locked except when a person is actually using the door. As such, you'd want another body in the cockpit to be able to unlock the door.

This brings up another question. Let's say it's prior to 9/11, and there's only one pilot in the cockpit, and they pass out. Assuming the auto-pilot is on, if they were to slump onto the control yoke, would this do anything? That is, does application of the yoke disengage or otherwise override the auto-pilot, or must it be specifically turned off before manual control input is allowed?