The why is pretty easy for anyone under 48 years old to understand; that is anyone who was 10 years old or younger when Ronald Reagan came into office.
Since Ronald Reagan's presidency, Capitalism with a side of Democracy has been the dominate sociopolitical direction for the country. With the fall of the Soviet Union, the mix has turned more Capitalistic with less Democracy and proceeded to be less Democratic yearly.
I don't want to argue Republican versus Democratic Party, but neither major part has produced great results for many of our people, particularly American under the age of 40. Getting a job, paying for education, moving up the socioeconomic ladders - all of these things are getting more and more difficult, while older Americans and richer Americans are doing better and better.
To these younger Americans, Capitalism definitely is not doing a great job of providing opportunities. So they look to a limited set of governmental intervention to provide goods like a $15 minimum wage - which would be a bit more than inflation adjusted minimum wage from 1978, or free higher education - which was the norm when I went to college in 1977 - 1980, or free health care - which seems reasonable since we spend more on health care than countries where it is all subsidized.
While to older Americans, this reeks of Communism and everything we fought against, to young people it reeks of European life style where people are happier, take more vacations and live longer.
Neither side is 100% correct here, but the Capitalistic side has been shown to be severely wanting, and you have to expect young people to want to try something, anything, else when the dominate system is systematically leaving them behind.
The push for a new system is almost assured when the old system is failing so many. And this movement will only grow larger and more self-assured the longer it is unaddressed or demonized.