Because when Barry first entered the speedforce he was poisoned by Thawnes negative speedforce (another yuck) and when Johnny Quick touch Barry he died. Crumbled to dust. Like Savitar did when Barry touched him. Hey now that I think about it maybe that means Johnny will be alive now? Since he died the same way Savitar did. And in Flash Annual 2021 it was revealed Savitar didn't die. He was sent to that speedforce prison.