Another reason is that if people only have one day to vote, it’s much easier to disenfranchise their vote just by delaying or inconveniencing them. Texas recently tried this by changing where people were allowed to vote, causing mass confusion.
https://www.houstonpublicmedia.org/articles/news/politics/el...
In Georgia they passed a law making it illegal to hand out bottles of water to people in line to vote while simultaneously reducing polling stations in certain areas to ensure long lines.