Since a kernel is something that is expected to run forever, it can't afford to leak anything over the long term. For most GCs, collecting that last little bit of garbage (in deterministic time) requires O(committed address space) memory bandwidth. A full-copy style GC may take O(object memory), which could be an improvement.

Now that memory and applications are routinely many gigabytes, this is a big deal.

It's hard enough for an ecommerce web server to maintain responsiveness, I couldn't imagine trying to respond to hardware IO interrupts in real time while running a collector like that.

Erlang (and its way) is a much better fit there, I think it'd be a delightful apps language: the GC runs at the (erlang) process level, each process has its own heap, so even though the GC is a vanilla generational GC by the magic of the Erlang VM it turns into a highly concurrent pauseless GC (only needs to pause a single Erlang process at a time, and you generally have tens of thousands chugging along).

Not so much as you might think. Both Smalltalk and Lisp were OSes early on. If you dig around in some early Smalltalk images, you'll find the 4 stubs for "put the drive head down" "pick the drive head up" "move the drive head out" "move the drive head in."