I think the main reason is, that Groovy seems to be abandoned, while Kotlin is in development, with JetBrains backing it.

But as far as language features support goes ... Kotlin, Ceylon, Groovy, new versions of C#, most of the times the language features are never that far apart (and they definitely do steal all features among each other) and you choose based on library and tooling support, familiarity and popularity.

Kotlin will come out among the top w.r.t. tooling support, because JetBrains will support it in its ide and library support because of its java compatibility. The android announcement pushes its tooling story even further.