Because it’s built around a very specialized set of needs (data manipulation, visualization, and statistical modeling), and it is essentially best in class at it, but it has quirks as a result. Anyone coming to R from a background in another language will feel those quirks intensely and assume it’s bad.

A lot of programmers dislike R for the same reason they dislike PHP: weak typing.

If you can get over that, though, there's a lot to like about R. It's sort of like a hybrid between a Lisp and an array language.