[caveat: I am a numpy/scipy developer]

The idea of rewriting a large body of code in a different language does not make much sense.

Also, being a niche language has some nice consequences:

  * R has been there for a long time through its predecessor S
  * R is a specialized language: little chance to see it being screwed up by some library which wants to change everything, as it happens too often in python
  * Because it is a niche language, its behavior is consistent across platforms (it is just easier to do with R than with python, or other "real" languages).

Note how being a "real" language goes against those advantages. Also, most researchers are very lousy programmers. Often, their software is super smart, but the code quality is awful and write-only. A less powerful language may mitigate those issues