Leading up to the 2020 presidential election, Donald Trump urged his voters to vote in person while Democrats promoted voting by mail due to Covid pandemic. Steve Bannon was secretly recorded saying that this was the Trump Campaign's plan to falsely claim the election was stolen. Steve Bannon does not deny saying this. By law, votes can't be tallied until polls close. Mail in ballots take longer to process because envelopes have to be opened, info needs to be verified then the vote can be counted. In person ballot results are faster because verification has already taken place and in many places ballots are scanned waiting to counted by a machine. Of course this is how it went down.

So to answer your question, Republican voters have a distrust for mail in voting. Largely because a president of the United States told his supporters to vote in person. During a pandemic.