Why do theorists place an upper limit of 500 solar masses on Population III stars?

Well sweetheart, those theorists ain't playing around when they say Pop III stars can't grow bigger than a potbelly pig. They reckon that once a star clocks in at about 500 solar masses, its gravity just becomes too dang strong for it to hold itself together, and it ends up throwing a cosmic hissy fit called a pair-instability supernova. So, to keep things from getting messier than a pig in a mud pit, they stick that 500 solar mass limit on those bright beauties.