I am sorry but you are missing the point of my question, it is not why Google did it but in comparison to Groovy.
I'm sure they would have also developed the build language from scratch also, and I doubt it would have been as cluttered as Apache Groovy. Check out the Groovy developers mailing list [1] to see how inactive Groovy development is nowadays.
[1] http://mail-archives.apache.org/mod_mbox/groovy-dev/201705.m...
People like it because it is a less bad Java. Most of the benefits with fewer drawbacks.
I personally prefer scala but the market doesn't seem to be going that way.