It's mostly the fact that individual lifestyle choices won't do anything (no ethical consumption under capitalism, etc).
Widespread veganism will never occur under capitalism, there's too much profit motive in the meat industry. As long as capitalists can exploit that industry, it's going to be there. Individual choices will not affect anything in a systemic way, being vegan is simply a personal choice that makes someone feel better. Same way that choosing one seller over another because it may be more "ethical" but won't push for any systemic change, or trying to lower your carbon footprint won't do anything for society as the onus of climate change is on capitalist producers and industrialization.
Side note, I've also noticed that while most communist vegans understand this, liberal vegans tend to be the stereotype of the over-the-top vegan who get stereotyped as pushing views down peoples' throats. They don't approach things materially, it's the liberal thinking that individuality and ideas change material condition instead of the other way around.