My guess is because Spohn makes one, BMR makes one, Hotchkis makes one, GM Performance Parts makes one, and Addco makes one. UMI has made a name for itself by designing (or re-designing, in many cases) quality parts, having really clean welds, and using high quality rod-ends and/or poly bushings. I'm a huge fan (I have their SFC's, lower control arms and ordered their adjustable torque arm from Sam Strano the other day), but I guess I just don't see the need for them to do a part whose design can't/doesn't need to be improved upon.
Of course, I say this and they'll probably announce next week that they've made a huge breakthrough in swaybar technology, just to prove me wrong.
-Mike