Is this really unpopular? Rightwingers certainly seem to agree with this.
That said, there's "not being vegan or vegetarian" and then there's "eat so much meat and dairy that it's significantly deteriorating your health", and the latter seems to be the standard for most people in wealthy countries. And as a species, we can't afford to eat like this if we aren't fine with mass casualties and being a global extinction event (vegan agriculture kinda already does that anyway, but it would be much less severe).