R core developers are notoriously resistant to change. A number of glaring inefficiencies have gone unfixed for years, despite people submitting patches etc.
R core is generally motivated by ensuring that R continues to work as is, not by improving performance. You can argue whether or not this is a good idea, but in the absence of a comprehensive unit test suite, it's pretty hard to improve performance without breaking behaviour.
It's difficult to see how this rationale can possibly justify ignoring a 10x speed up in vector-matrix multiplies (and similar speedups for some other matrix multiplies) that can be achieved with a modification affecting a dozen or so lines of easily-checked code.
