Scala and Groovy serve very different needs in the Java ecosystem. The original creator of Groovy, who hasn't been involved for quite some time, had an itch to create a language for the JVM because he wasn't satisfied with what was available. So, he began working on Groovy. Several years later he came across Scala and liked it. So, he stated (I'm paraphrasing) that if it had been around back when he as looking for an alternative language for the JVM, he would have used it rather than create a new language.
I haven't looked at Lift, the web framework for Scala, but I would hazard a guess that it doesn't support many of the features of Grails. Many of the most powerful features of Grails are available because of the dynamic nature of the Groovy language.
It's also worth noting that the Groovy language has come a long, long way since James Strachan created it. It took on new leadership, a new team of incredibly smart developers and significant investments were made it in it by a few different companies. It continues to get faster and the integration with Java keeps getting better.