Lack of GC makes concurrent lockless programming extremely hard (and most of the examples on the internets are subtly broken). RAII doesn't work at all besides trivial cases when there is no sharing at all. Reference counting incurs additional contention, due to interlocked increments/decrements and is also very hard to do properly. Where in a GCed language a single CAS is often enough, in a non-GCed language you need to resort to less efficient and often completely unsupported things like DCAS or CAS2.