It pretty much goes like this:
1)GOP realize that there are more people who will not vote for them than will vote for them in the US due to their terrible policies that harm the people. Instead of changing their horrible policies that are causing this, they opt instead to utilize gerrymandering software to divide the voters up to make it more difficult for the GOP to lose, but since that isn't even enough on it's own, they instead resort to voter suppression.
2) GOP shut down polls in democrat regions, make sure the only ones they leave open are difficult to access and people have to wait in unreasonably long lines to do so while at the same time allowing voters in GOP districts to have easy and abundant access to polls, often resulting in being able to vote in 15 min or under as I can do here. There are 2 polling locations located within 5 min of my home, many more within 15 min. In addition to removing access to physical voting locations the GOP then also repeatedly remove democratic voters from the rolls so they will not be able to vote at all and will have to reregister to vote before being able to do so. They also attempt to implement other measures to prevent demographics they have determined to be less like to vote for GOP such as requiring them to have an individual postal address, making it difficult for the Native Americans living on reservations, those forced to live on land that is without street access and homeless Americans to be able to vote. The requirement to have an unexpired photo ID also prohibits those with medical conditions that make even being able to go to the office to obtain one life threatening or impossible to physically do. You have people with suppressed immune systems that cannot safely go to those facilities, people who are literally bedridden, hospitalized or have other conditions that prevent that from being feasible will not be able to obtain a Photo Id easily , and such a requirement removes their right to vote. Often the same obstacles preventing them from being able to go obtain a current Photo ID ALSO prevent them from easily visiting their physician to obtain whatever physicians letter that state may require to exempt them. The additional hassles on top of their already daily struggles prevents them from being able to vote at all. These voters currently usually either vote by mail or do not vote at all. Then we get around to the whole, " remove" their right to vote entirely by increasing the charges against them for drugs to felonies because they see that more democrats as recreational drug users are arrested than republicans due to the over policing of minority communities, and then not restore their ability to vote after they have served their time. Of course we then additionally have " voter intimidation" that is associated with that where they try to " scare off" minorities and other marginalized groups from the polls by threat of being arrested for recreational drug use and outstanding traffic tickets and such on top of everything else they have already done. All of these obstacles combined add up, and in the end it results in many voters not being able to vote due to their votes being suppressed by those who want to prevent them from voting in the first place.
3) Many republicans, including Trump, have enjoyed the convenience of being able to vote by mail for many years and republicans still encourage people in republican districts to do so.
4) Allowing Democrats to avoid going to the polling locations by voting by mail allows voters to avoid the voter suppression and intimidation that has been so successful for republicans to reduce the amount of opposition voting at all over the years so they panic at the idea that democrats could actually avoid all the obstacles they have placed to prevent them from doing so and start screaming " Voter fraud" when their own data shows this to be inaccurate. They then support a disinformation blitz supported by Russian trolls claiming voter fraud is rampant in mail in voting even when all the evidence we have says otherwise. Vote by mail undermines the GOP ability to prevent democrats from voting and the GOP is doing everything they can to keep their voter suppression obstacles in place.
5) Instead of accepting that vote by mail increasing the access of democratic voters to be able to vote, Republicans then move to attack the post office itself so that they can break the postal system to prevent the mail in ballots from being counted. In addition, at the same time, we have the GOP president then going online and telling people to illegally vote twice because then he worries that his intentional effort to break mail in voting would also impact his own voters mail in ballots, he then goes online and tells his supporters to commit voter fraud so he can contest the results of the election instead. He would rather have his supporters go to jail than accept a fair election in the US.