Billy:  

Because UNA has unique features which makes it very attractive to work with. The only thing about UNA is that she is difficult to work with. If you look at WordPress coding, it is very very well documented in the comments. That is part of good quality coding.
What WordPress offers is - BuddyPress... I tried it... I didn't like it. UNA in comparison is like a fine Portuguese wine in front of a fireplace with a high-class call-girl. Buddy Press is more like the homebrewed beer of the hillbilly fishing party. I could go on, but I think you get the idea.
Any high-class call-girl needs accessories, But UNA does not have those. Wordpress have high quality accessories their hillbilly girls get to wear...
Combine Beauty with beauty and you get something like an Amazon Goddess...