Regarding the Evac, its not always safer outside the plane than inside....Its possible to have engines still running at moderate throttle & other external dangers....
An assessment needs to be made (quickly) as to the threats & then act.
Not as easy as it sounds in the moments after a crash however.
I also suspect that the default answer by airlines is to stay in the plane as the safest place in most circumstances & they need a strong reason to override that. It was clear that the stewardesses by procedure had to ask the pilot for a decision no doubt as their training advises them to do so.