There was a thread about this on the ST forums a while back - I can't seem to find it now. If I recall, block collapsing was a low/non-prority because it was viewed as lending itself to poor code, and mostly compensated for by other features.
My personal opinion - it'd be nice, but I don't miss it enough to switch editors or bug the author - maybe someone will make a plugin for it. I haven't missed it at all in Python or Haskell. Last time I worked on a Java project, I missed it a bit at first, but mostly found that the minimap got rid of that feeling of looking at code through tunnel vision that made me want to collapse it, and by the end of the project I'd shifted to using ST more than Eclipse. I might feel differently if I jumped back in to C#, with #region and all, but I really feel like the C language family lends itself more to IDEs anyway.