You say that with intent to imply it must be a bad reason, it's a little underhand. There are lots of reasons Groovy is not well appreciated. It's name is terrible, the community has always had a somewhat amateurish image (in stark contrast to the actual professionalism of what they do), it's choice to blend in seamlessly with Java and adopt as much Java syntax as possible made it decidedly unsexy with the crowd want to push new language concepts. It got a bad reputation for being slow early on (quite undeserved since it was always about as fast as comparable languages).

I think it's actually used incredibly widely but people don't even know it, Gradle being an example, but lots of other similar cases. Groovy just doesn't get credit a lot of the time because it embeds so seamlessly in to the Java world.

I think this due to Groovy's fragmented ecosystem (Groovy's 2 big recognizable projects, Grails and Gradle, don't play well together) combined with the fact that you can't use Groovy everywhere you use Java.