At least 157 people were killed and 270 were injured last year in unintentional shootings by children, according to Everytown, an advocacy group for firearm safety.
People in urban areas--which is most of the country's population--almost exclusively experience firearms as being part of a criminal act. Most people that live in cities don't know people that hunt, or compete in marksmanship, but they hear about murders and shootings in their city all the time. Why do you need training in firearms in schools when the only use--the only use they have consistent exposure to--is criminal?
You can look at electoral maps and see this; most of the geographical area is red/Republican/conservative (typically pro-2A), while most of the population centers where people actually live are blue/Democratic/more liberal. If you went back 50 or 100 years, you'd see more people living in rural areas, which ended up meaning that there were more people that were exposed to hunting, etc.