US election: early voting

It used to be that most (all?) states allowed early voting for people who had some good reason for it. Lately some states started allowing it for everyone who wants to vote early. I just realized that there are 30 states now that allow early voting, and that they are distributed geographically in a strange way: all states in the Western half of the US allow no-excuse early voting, and very few states in the Eastern half of the US do.