Environmentalists aren't just environmentalists. Everything became extremely politicized over the last 15 years in a way that it wasn't back in the 90s when environmentalists were talking about overpopulation. Nowadays, if you think of yourself as an "environmentalist", then you're on the "left", and if you're on the "left", then you're overarching worldview is based on "social justice" ideology. And the primary concerns of social justice ideology are about what they perceive as racism and sexism, with environmental concerns far down the list. Even when they do talk about environmental issues, they mostly talk about "climate justice", which mostly means transferring money from rich white people to poor non-white people as a way to punish white people for having polluted more in the past, rather than doing anything to actually reduce environmental damage. And any mention of overpopulation is inherently considered deeply racist, since overpopulation is primarily a third world problem, whereas social justice ideology teaches that third world cultures are perfect and all the world's problems come from the white man. Therefore talking about overpopulation is anathema to social justice people, and by extension to modern environmentalists.
To get environmentalists to talk about overpopulation again, you have to do a lot more than just remind them about it. First, you'd have to turn the modern reality of society up side down, breaking down the way that people have sorted themselves into broad ideological camps, and allow individual issues to decouple from the overarching identity politics that has taken over society. In other words, not gonna happen any time soon. I don't know what will drive the next major cultural shift that gets Western civilization out of its current fall into social justice ideology, but so far we are still falling in deeper, not yet climbing back out.