> Oddly, many modern scripting languages use ancient compiler technology

Historical: most of them are 10~20 years old, "new compiler technologies" had not trickled down much when they were created and they had no real need for the things those provided (not to mention they have drawback, a GIL has better single-threaded performances than fine-locking which is important when single-treaded is your primary workload).

Also manpower, probably, it's harder to implement a generational concurrent GC than a refcounting or a basic M&S, and it's even harder to retrofit that into an existing codebase (hence pypy having pluggable GC backends and defaulting to a hybrid GC)