The reasons are obvious once you try them on.
They're light, super comfortable, accent your legs really well and it doesn't hurt that I've gotten the most compliments about my legs while wearing them (it's almost like they make my nonexistent calves...not be the focus anymore).
These shorts have a dedicated following and I completely understand it. I tried wearing a pair of “normal” shorts the other day and I thought I was in a Limp Bizkit music video.
Shit was hanging loose, down to my knee, looking boxy. And all the attention was therefore placed right back on my calves.