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Mostly because the huge gains in speed that were desired were found in the native compilation fork.

That, and the R5/R6/R7 debacle has alienated many from scheme.

Not to mention the humongous pile of Elisp written over 40 years of its existence is hard to be backwards compatible with for any new implementation.

Emacs' core isn't known to be the best codebase in existence, but it is battle hardened and hard to replicate.