The simple truth is that the GOP had a disastrous 2018 election cycle even though they picked up a few Senate seats. They should have picked up at least 8 of them because it was very much their year to knock off a lot of Democrats in GOP states.
I just looked. Looks like Republicans have 22 seats up for grabs in 2022. BUT, most are in the deep South or the farming Midwest. Plus Alaska: I am wondering if Palin takes on Murkowski.