You have a good hypothesis there. I wonder also if possibly the studio figured too fast a fire was a hazard they weren't willing to tolerate --- this was all shot in studio sets, sometimes painfully obviously so --- and figured that fire would just have to do by itself. Of course, studios were still using stuff like that spray-on foam that was basically accelerant with air bubbles through to the mid-70s, but they did at least try sometimes to keep the explosions down to a tolerable number.