Because Scala still has a lot of the verbosity of java. So I'd rather use plain java for the plumbing, and then use concise, clean, reads-like-documentation short jython scripts for high-level programming.
[edit: by which I mean I don't have such a need for properly functional programming with function-as-argument etc. Most of what I do is plain ol' number crunching, paired with threading.]
[edit 2: i.e. "blub" is better. I have to give scala a better look one of these days.]
If I were in an arguing mood, I'd say that in terms of verbosity, scala is closer to jython than it is to java... the only major number-of-characters difference is that you need type annotations for method arguments (and return value if it's recursive).
Still from a 'conciseness' point of view and ignoring all the benefits of closures, first-class-functions, and other goodies, scala still has some serious awesomeness to serve up, especially when navigating the world of Enterprisey Java Libraries: implicit conversions. A good explanation is here: http://www.artima.com/weblogs/viewpost.jsp?thread=179766
There are also some downsides, of course ... I'd say they can all be summed up by the facts that scala is a young language, still evolving, and it is a large language with lots of features (some rather hard to grok at first).
[edit: by which I mean I don't have such a need for properly functional programming with function-as-argument etc. Most of what I do is plain ol' number crunching, paired with threading.]
[edit 2: i.e. "blub" is better. I have to give scala a better look one of these days.]