Not exactly sure why. I did notice that I couldn't solo them after 50 (on a dps toon), despite being decently geared, although I admit I stopped even trying. Obviously, there was nobody else doing them or I would have joined them.
I suspect that the problem is that they were designed to be group content, but there are better things for groups to do, which is cool.
However, it seems a waste of content, seeing so many post-50 maps without any progress made on them.
I'm guessing that the rewards for doing them just aren't sufficient compared to other things that level 50-60 toons could be doing, which is cool.
But it'd be nice if the rewards were sufficient to make them worthwhile.