My guess would be that so many CTFs have moved away from PHP partially to help provide practice against newer applications, like the million different JS frameworks that are hot these days. Could also just be due to the fact that so many PHP-based challenges already exist; perhaps it’s easier to come up with ideas for something different with newer applications. Not saying there’s anything wrong with PHP-based challenges, of course — they’re relevant too — just that I could see the reasoning for avoiding them.
(Also, only 600? That seems generously low.)