Cost, frame availability, added weight - they're a bit of a niche product. Traditional derailleur shifting is pretty damn good nowadays too. Some people are dead set against the grip shift as well even though it works well, the newer electronic shifting has fixed that though.
There's a big push for downhill teams to use pinion gearboxes and belts this year and a few big teams will be using them, hopefully that'll give them a bit more exposure.