Because they rock. ?
If it wasn't the Bison, I would have went with a Rubi Gladiator (and may yet in a few years).
People buy them for the same reasons people buy Rubicon model Jeeps or up-fitted other trucks and SUVs I suppose.
Turn key (mostly), factory warranty, etc.
There isn't anything on the Bison package that isn't practical (embroidered seats and some badging aside), and the aftermarket goodies that AEV and Chevy have come up with are well thought out and executed.
There is still enough to tweak here in there however, so there is still plenty to wrench on.