Those comments are unpopular because most people can see they're just smug self righteous twitter tier mic drops.
I don't see how this is compatible with the observed reality that individuals make choices that are not best for society (whether due to rational self-interest/incomplete information/behavioral factors).
The former is hazardous (and verylittlemeat spoke against it): people fight for their views on the internet, and it degrades discussion more than it reaches people. The latter has leverage and is potentially highly impactful (i.e. the kind of discussion I come here hoping to find).
They're totally reductive and really just serve to stroke the speakers ego or shame others rather than actually address the issue.
And the (not always, but frequent) I'm-holier-than-thou-because-I-transcended-ape-instincts vibe doesn't help.
1) Because you're treating an idealized version of "the environment" as a terminal goal.
2) The people most likely to agree with your argument are precisely the ones who should be having more kids.
I would suggest that whether an idea is good and whether it is popular are mostly orthogonal, but also that encouraging people who aren't certain they want kids to not do so is, from the perspective of someone concerned about climate change, a good idea.
Interestingly advocates of veganism (which has a similarly massive impact on your carbon footprint) are much more welcome, but I guess for most people the drive to reproduce is more inherent that the desire to eat meat.