My research suggests a key reason why this happened: our partisan identities motivate us far more powerfully than our views about issues. Although voters may insist in the importance of their values and ideologies, they actually care less about policy and more that their team wins.
This “team spirit” is increasingly powerful because our party identities line up with other powerful identities, such as religion and race. Over the last few decades, Republicans have generally grown increasingly white and churchgoing, while Democrats have become more non-white and secular. This sorting of identities makes us care even more about winning, and less about what our government actually gets done.
This helps explain why all of the five states noted above voted for liberal policies even though they have substantial proportions of white churchgoing Republicans. Indeed, Nebraska, North Dakota and South Dakota have some of the highest percentages of white churchgoing Republicans of any state.
When social and partisan identities align, we begin to detach our votes for candidates from our policy interests. The most important thing is to stick with the team. It doesn’t matter if the team you voted for opposes the very policy you voted to enact.