Perhaps because they take those features for granted because it uses some GC provided by LLVM ?
(I don't know anything about LLVM support for GC, but I would be surprised if it comes with a GC that doesn't require to do something special in order to make it concurrent)
LLVM GC support has been fairly weak, however Apple has been working on using LLVM in JavaScriptCore (FTL project), which requires that, so it should be getting better. I have no idea if this project is related to that in any way.
