Pluggable schedulers have been proposed, implemented, and shot down by Linus several times in the past. IANAKH, but Linus's argument seems to basically boil down to this: for a monolithic kernel, delegating something as central as task scheduling to pluggable modules is a pretty big hit in terms of latency and complexity vs. just putting the best, most tightly-tuned scheduler you can smack dab in the heart of the beast.

: I Am Not A Kernel Hacker