English-speaking culture is steeped in Christianity (even modern 'wokeism' is just another, more radical iteration of calvinism), and Christianity is a doomsday cult; we're fascinated by doomsday narratives, the end of the world, mass death- we're in love with these stories; look at zombie movies etc. The overpopulation meme is certainly going to be partially fed by how appealing 'the end of days' is to the western mind.
(Right wingers have a doomsday narrative, too, it's just a biosocial one rather than an ecological/geographic one; modern technology and the resulting societal shifts lead to selection against intelligence, and societal decay propagates irreversibly, leading to a future of stupid, lazy, and most importantly small-souled bugmen.)
The people that I do see promoting this sort of population limit theory are new age hippies, vegans and the like. Recently seems very popular with more college educated folk. 'How can you think about having kids when you know how bad it is for the environment'
Well that's an.. interesting take