I read it as comparing the three exposures to each other.

The first evolution, Charmander was exposed to lead and the toxic effects of lead are pretty fast acting. Meaning they were weakened the quickest and couldn’t evolve.

Charmeleon was exposed to asbestos which has latent toxic effects, they show up decades after exposure. So the Charmeleon lived quite a while before they showed any noticeable effects.

The final evolution was exposed to micro plastics and because, so far, micro plastics seem to be so unreactive they just chill doing nothing the Charazard had an opportunity to evolve.