You've assumed that statically typed languages are somehow "better" than dynamically typed languages and then asked "Why doesn't everyone use the better languages?" Dynamically typed vs. statically typed is still an open question and is a lot more complicated than "which is better?"

> And on the plus side, you get all the benefits of compiler type checking, better IDE support, and great refactoring support.

Since when? Compile time type checking is only a benefit if you're worried about type errors and in my experience the people that are most concerned with type errors are the proponents of statically typed languages. With respect to IDEs and refactoring support, my experience has been more along the lines of "C# and Java have good IDEs and refactoring tools, everything else not so much." I personally find Scala and Haskell's tool support to be far behind Python's, just for example. Also, Smalltalk has, arguably, far better tools available than any statically typed language (or any other language for that matter).

Nobody has proven that statically typed languages are better than dynamically typed ones, nor that applications built with them are less error pronen. I would argue that the amazingly widespread use of Ruby and Python and the average quality of projects written in those languages is evidence to the contrary.