quarantine 'em

OK, so, this is my theory, although I doubt it is solely my theory. Edna Mode is the driving force behind the entire Incredibles universe

Edna Mode provided costumes to both superheroes and supervillains

When the government clamps down on superheroes, she stops producing costumes entirely, which is why supervillain activity dries up too

She provides Syndrome with his cape, fully aware of its weakness and intending for him to be defeated

She does this to kickstart a new age of superheroes. Out of boredom with "super" models

She accurately predicts Jack-Jack's flame powers, despite him presumably having tested negative for superpowers at birth