In the case of The Warriors I suspect the original premise sounded exciting. Got producers interested. They started filming. Then somebody got a look at the rushes for the Rally Scene. Slam the breaks on that scenario!!
( I don't know why they are so paranoid. Just have the gangsters get the upper hand in the beginning, then the National Guard wipes 'em out. )
But that's just my conjecture. I haven't read it in any Hollywood histories or anything.