Because they already have it on-site and track installation is one of the easiest parts of the build. Since this is a "new" ride system, it will certainly need a decent amount of test and adjust time, but before it can open at all, we also need to get the Guardians together to film for the ride... which was why this was slated for 2021 - to coincide with filming of GotG 3. Now that Disney screwed that up, it no longer will be convenient and they will likely have to schedule a day where everyone is available for a few hour shoot.