I think that vegan diets never took off primarily because the vast majority of people do not want to follow vegan diets. Money factors such as subsidies are secondary. There are also potential health impacts. I followed a vegetarian diet for a few years when I was younger because of ethical concerns, and I felt healthier once I started eating meat again. This is just anecdotal evidence of course, and many people argue that you can be as healthy or even more healthy vegetarian as you can be omnivorous. But it's a potential psychological factor arguing against the likelihood of mass adoption of veganism.

Mass transit and dense development are potentially doable, but at this point the suburban sprawl + car model has been popular in the US for decades and it would feel like a downgrade for many people to live in smaller places in more heavily urbanized environments. I think that many Americans don't even have a clear idea of what it would look like to raise kids in a multi-story residential building in a city without it seeming like poverty.

Also, in some parts of the US, mass transit and dense urban living run into the problem of anti-social behavior including street violence. Many of the people in the US who care the most about stopping climate change also oppose the kind of intensive policing that would be required to keep crazy dangerous people off buses and clean up troubled neighborhoods enough that the average person would feel safe living there. Violent crime rates have largely been going down since the early 90s, but are still much higher than in Europe. One of the reasons for the high rates, through definitely not the only reason, is the availability of guns, but getting US right-wingers to accept gun restrictions is probably even less probable than getting US left-wingers to accept major crackdowns on street crime.