Actually, it’s less Democratic than exactly half its neighbors. It voted for Trump twice, while Minnesota, Wisconsin, and Illinois all backed Biden.
I guess it’s safe to assume you’re asking “why is Iowa so much more Democratic than SD/NE/MO?” or “why is Iowa so much more Democratic than other Plains states?” It’s more Democratic than the sates to the west because it’s much more densely populated, with less ranching. Also, while Iowa is not as rural as South Dakota, it’s not as suburban as Nebraska (though as a result of trends in the age of Trump, IA and NE are converging somewhat). As for Missouri, Iowa’s neighbor to the south is for the most part a Bible Belt state (when I was in northern Missouri, the accent sounded Southern-ish to my ears), while Iowa is not a Southern state at all.
Is Iowa a Plains state?
