Scala implement tail call optimization in the compiler. In general the JVM has always outperformed the CLR:
http://benchmarksgame.alioth.debian.org/u64q/benchmark.php?t...
http://benchmarksgame.alioth.debian.org/u64q/benchmark.php?t...
.NET is Windows-only, which is a significant limitation compared to the JVM. I don't think better performance on Windows -- even assuming going to CLR instead of JVM would provide that -- is a good reason for Scala to move to the CLR as the primary target.