Because it's cheaper than decent name brand cat litter for how many cats we have and boxes I fill. It lasts way longer, it is less dusty than most clay litter, and it has no fragrance that sets off my severe allergies. It clumps as tight or tighter than many clay litters I've used, making the boxes cleaner and fresher. And when I dump the entire box to clean it, I can rake it into the leaves in the woods on our property and it doesn't end up in a gross pile for months on end like clay. We also had a cat with a skin allergy to the dust in clay litter and her belly broke out something fierce. Again, that totally resolved when we ditched the clay. For us, it's proven to be the best option. I read about it on a cat rescue's website years ago, others I'd never have even thought about it.
