I don't know of a definitive study that controls for all variables but one theory is the heavy Nordic population there.[1]
If you believe in cultural factors... (e.g. memes such as "Swiss punctuality", "German engineering", "Protestant work ethic", etc), there's something about Nordic culture or solidarity that aids success. (E.g. Norway does not squander its oil resources and built up its citizens' pensions -- while oil-rich Venezuela is basically bankrupt.) It's hard to think of a geographical area in USA with significant # of descendants of Sweden/Norway/Denmark being an economic slum.
It's an interesting correlation, if not provable causation.
I would assume its because the majority of Minnesotans live in the Twin Cities Metro and the Twin Cities is doing pretty well economically. For example 16 of the 17 Fortune 500's in Minnesota are headquartered in the Twin Cities and the unemployment rate is about 3%. If you drive outside the Twin Cities lots of towns look pretty distressed.
By bucking the trend I mean that compared to most other states it appears to be very comfortable and prosperous across the state in most but not all counties at least according to this map. I’ve seen similar findings in New Yorker and in the NYT too.
Edit: is there something MN is doing that we could all learn from? They weren’t apart of the North Dakota oil boom really so I don’t think it’s that.