Around the time that GWT came out, offshoring was a big thing. And most of the contractors only knew Java. Also Java was the trusted language and javascript was not.
The only big GWT project I've ever been on was a governmental project that I won't go into (because it's Danish and I would have to describe a bunch of stuff that everyone in Denmark knows and nobody outside would probably care about), but the company providing it was porting their Java version to JavaScript and had a significantly large codebase to leverage.