I reckon that'd be because agreeing on what is "better" than something else would be rather hard, considering most terrainers have their preferable ways of using the doodads they have. That, and some terrainers don't really like using "high detail" doodads, due to maintain that "wc3" look to it.
I have to agree though, setting together a better mix of more useful doodads, and rule out the absolutely useless doodads would not only, likely, make the map smaller in size, but also more efficient and better.
All this aside, I still see little point in this, people will always make their own "UTM's" and stick to that. I guess the "UTM" is really only very useful to new terrainers and people who don't terrain, but like fooling around with terraining.
- Which happens to be a diminishing race.