Perhaps as importantly, R has significant buy-in in the statistics community, so a paper on a new technique will often be accompanied by an R package implementing it; in fact several journals explicitly prefer R packages for accompanying code, because the reviewers are likely to be familiar with how to use it. That's partly due to its semi-continuity with Bell Labs S (http://en.wikipedia.org/wiki/S_(programming_language)), I believe, which was a language designed by-statisticians-for-statisticians. It's fairly hard to replicate that; would require considerable effort to migrate the whole community to a new consensus environment.