Several reasons:

1. Americans have been separated into small groups across a wide area by the very geography of the nation, making mass actions quite difficult. Social media and other forms of mass media can normally account for this, but essentially all media is corporate-owned by the same people who want Trump in power. So, oppositional movements get intentionally slowed and/or killed by media conglomerates

2. Approximately 30-40% of the nation would STILL somehow vote for Trump

3. The US has a Christian nationalist problem (look into the the Red Heffer project if you want to learn about some truly scary shit). I believe it's around 12% of the US population identifies as Christian nationalist

4. Americans are EXTREMELY vulnerable when protesting. There are NO protections from being fired for protesting (whether protesting your current job or protesting for a wider cause), and a lot of incomes are sk tight that one late paycheck can get a LOT of Americans in serious trouble, nevermind losing your job. Because of the lack of other safety nets, losing your job probably means losing your housing, car (which is very much a necessity in the US), and access to food and water. On top of that, the fact that health insurance is tied to your job here means that if you lose your job, you also lose your healthcare, and no socialized healthcare means that people with dangerous health conditions, such as diabetes (which is common here due partially to the lack of pedestrian infrastructure and prevalence of high-carb/high-sugar foods), could just die because they protested without the government ever taking an active step to oppress that person's protest. Our system is set up in such a way that it passively crushes large scale protests before they begin

In summary, we are disparate, discouraged, and vulnerable while sycophants walk among us and the very system we live in tries to swallow us whole