[mst]

Honestly, probably because so many languages have non-compacting collectors that people just accept it as a trade-off - in the sense that compacting collectors are non-trivial and without lots of work can produce significantly higher GC pauses, and so doing it really well requires a lot of engineering effort that you might prefer to be spent elsewhere adding features you want more.

golang's collector isn't compacting either - though it uses per-size-class arenas for allocation so you don't end up with fragmentation bloat to nearly the same extent. Part of me wonders if simply building R against jemalloc would get a decent chunk of the same advantages.