Why on earth would the pilot not order an immediate evacuation as soon as the plane came to rest? They crashed. Planes that have crashed tend to catch fire. So get off the plane!!!
You must not be a pilot? Evacuations can and do cause injuries; you don't evacuate until you complete an assessment and verify that evacuation is a safer course of action. It would have taken a moment or two for the flight crew to realize the amount of damage to the rear section of the fuselage - by all accounts, things were much better up front. Yes it was a very hard landing but until they have gathered at least the basics they aren't going to magically know the extent of the injuries and damage.
Perhaps the following B777 evacuation checklists will help:
http://www.aaib.gov.uk/cms_resources...%20App%20B.pdf
As you can see the Boeing version of this doesn't have the evacuate call (step 5) until after a few other steps are completed, which will make the evaucation safer for all involved.