That's because Marx was a productivist, and communism was strictly productivist until a decade ago (and still is mainly productivist nowadays).

In a lot of country political ecology used to be liberal/capitalist (save a few radical feminist like D'Eaubonne who linked environment with feminism, but it's less than a minority). Basically Blair's 'third way' but with less nuclear (for some reason, although I think this position is loosing ground in ecologist), and more electric cars.

The degrowth movement is an offshot of that ideology. Degrowth is to political ecologism what anarchism is to communism, based a very Idealist and hopeful view of humanity.

Communists and ecologists are broadly on the 'left', but rarely allied until maybe a decade ago, and again, on minor things (Communists love nuclear,as it is typically something you don't want a capitalist with 'limited liability' to take care of), and while degrowth might be close to anarchists in some way, it is very, very dishonest to put them in the same basket.