It’s the convenience factor. That’s it. Once plant-based options are more convenient than meat, game over. Most people lack the discipline to sustain a vegan lifestyle, even in 2018.
It's also indoctrination. We've been indoctrinated to think of meat as a staple food. Meat was never a staple food before factory farming came along. Fortunately, it seems that humanity's honeymoon with the farming industry is coming to its end.
Generally agree, although I think this is slightly the wrong way 'round. Meat was definitely a luxury, as recently as a century ago. (Speaking of the U.S. here.) Then we had an explosion of a (relatively) wealthy middle-class, who could afford lots of things that were formerly luxuries. And those things became expected, they became part of the culture, and they became commodified. I don't think there's any nefarious conspiracy, it's classic tragedy of the commons: everybody individually wants steak for dinner; the social price doesn't become clear until much later.