Because blue districts overwhelmingly support it and want the convenience. Because many hourly workers, in many of the lower-wage jobs, in the service industries for example, don't have the option to take off of work or change their schedule to vote. There are a lot of those voters in blue districts (less in red districts) and a lot of those voters are racial and ethnic minorities. It's blue vote suppression plain and simple. The people who turn out to vote are the people who have the ability to do so, to wait in line, to drive, to take that time. Voting by mail is inclusive.