Skimming this issue, it seems like they weren't expecting to be able to use this GC. I know C# couldn't either, at least based on an earlier state of the proposal.
this thread confirms my suspicions. some languages may benefit from a built in GC, but those languages probably use a generic GC to begin with. wheras any language that has a highly optimized GC for their own needs won't be able to use this one.
Skimming this issue, it seems like they weren't expecting to be able to use this GC. I know C# couldn't either, at least based on an earlier state of the proposal.