Why didn't the Whirlwind I use a high persistence crt or something ?
Because then no dynamic display, like the ball shown at minute 11:0, could have be done?
On this video Whirlwind I at 1:48, you can see the text being diplayed, it is only very briefly visible, apparently they would take a film picture and some hours ( or a day ) later see the results on the crt screen by looking at the developed film photo
The oscilloscope wasn't meant as primary device to display text or textual results at all, that's what the mentioned Friden Flexowriter, shown at minute 10.36 is about. It can produce printout or store results on paper tape for later processing/printing.
Developing films wasn't a matter of days, but an hour at most - keep in mind, that was right after the war, when capacities had been build to do so in minutes.
As said, the oscilloscope was meant to show (simple) graphics. Adding a camera to one was a standard method at the time to get a 'hardcopy'. To simplify/speedup handling the Whirlwind got two, one or direct view, the other for recording a picture when needed.
It's important to keep in mind that the Whirlwind wasn't any production machine like later, but an early experimental device, created to figure out the very basics of computing. And trying to show a 'full screen' of text is one of the experiments. Simply seeing if it could be done. The full result is visible at minute 11.36. For any useful text output printing it on the Flexowiter would be way more convenient, wouldn't it? Users staring 24/7 on a screen is a way later development.