It's one of the most rural swing states, but that's not the whole reason Dems have struggled there. The state has plenty of growing suburbs that in theory should blunt the rightward drift of the rurals.
Iowa has among the strongest incumbency bias of any state — voters like to keep incumbents, both Democratic and Republican. That's why its partisan voting index varies a lot more than, say, Florida.
This means Republicans have won Iowa a lot more than raw partisanship would predict.
But it'll be one of the harder states to win back, at least until the national environment and incumbency lines up just right.