Turbocharged,
Their has to be a chain of command, PIC orders evacuation. Believe me tech crew also have everyones best interest in mind.
Imagine what would happen if cabin crew initiated an evacuation before tech crew had shut down engines.
Good chance of a number of people being ingested into the fan blades. Human flesh and RB 211 blades spinning at several thousand RPM can only end up being nasty. RB 211 winning every time.
Tech crew have communications with the tower and a good overall idea of the situation and exactly how critical the situation is.
Evacuation may not be necessary.
Cabin crew that panic at a critical moment could possibly cost lives. Hence all the training and simulations. Human nature being what it is, some will panic. It's only when we are put to the test that we find out exactly what we are made of. With good training we can attempt to eliminate or at least neutralise the fear of the unknown.
Cabin crew can initiate an evacuation, but that is only after all attempts to contact the flight deck has failed (scenario is that they are dead) and it is imperative that everyone needs to get out fast.
Remember,the aircraft does not have to be in a situation as dramatic as you see in the movies.(all smashed up on the runway.) Evacuations can occur even once you have reached the terminal.