LogFAQs > #978739338

IIRC it was actually one of Kamala's creators who requested that she not stretch in the show, both because it always looks shitty in live-action and because they didn't want to give internet perverts an excuse to sexualize a minor based on her superpowers.

It's also possible that they changed them because they're introducing the Fantastic Four soon and didn't want to invest the money/CGI into two stretchy-people, and figured Reed is by far the more recognizable and iconic character with those powers. Or they wanted something that would let them give her a power more similar to Carol's so The Marvels could work as a concept.