Why is Minnesota so liberal?

[Nyvin]

Very large Nordic population (I think around a third of the total state population), which leads to support for left wing policy like bigger social safety nets, progressive taxes, and workers rights and so on.  

Minnesota is also highly educated compared to a lot of other states, which leads it to being pro-science on things like evolution and climate change.     Also this helps (I think at least) with it's very high voter turnout rates.

In the past the farmer's unions used to lead to a strong showing for the Democratic Party, but that's kinda fading, I think Collin Peterson is a remnant of that age.

Finally,  The Twin Cities area makes up about 60% of the state's population, and that leads to support for big government things like infrastructure projects, public transit, public education, and so on.

[Nyvin]

Minnesota actually has a very close PVI, and, given its whiteness, it could become much more Republican soon.

Minnesota has trended leftward in the last decade if anything.