The perfect evac is a co ordinated event between the Captain and the Fire Commander on the outside. These guys are trying to control the fire with foam, while the pilots will shutdown all engines/apu and prepare for an evac.
From the cockpit it is very difficult to gauge the extent of the fire, so reliance on input from the fire commander/cabin crew is an important part of building up the picture to evac or not.
The initial response team may not have the manpower to fight the fire and manage the passengers, so it is a lot easier to manage the situation without distressed passengers coming down the slides and running around without being marshalled into a safe area.
That being said, if things are getting hot/smoky in the cabin, there may not be the time to wait for more manpower on the ground.
I was once shown an old movie by the RFS of a DC10 in the USA with a similar uncontrollable fire, the skipper was pleading to evac the pax, the fire commander kept asking to wait so they could control the fire and get more personnel, they eventually did evac, the aircraft continued to burn, and it was a very successful outcome (except for the aircraft, it continued to burn for 20mins)