Early Voting = People who work in jobs lacking the autonomy to come in late/leave early on a Tuesday to vote on Election Day
Absentee Ballots = Elderly people who lack transportation
The above groups tend to vote for Democrats.
It's also a factor of strategy. Democratic voters tend to be the lower turnout voters so the marginal utility of early voting as a strategy is higher. It's essentially extended GOTV.
Democrats in states like Iowa also tend to have strong grassroots energy. Whereas the Republican strategy is more of a top down direct-mail, radio ad and television strategy. That's partially because their rabid base voters are more likely to write a check and less likely to engage in direct field organizing like canvassing a college campus or a county fair. That's especially true in Iowa where the Presidential campaigns create this institutional memory about effective field organizing and GOTV.
