Part of the deal is the cost. Gel cells typically cost more, and many people think more expensive equals better quality. I really believe that if AGM batteries cost more, they would be used more.
The other reason is that Gel cells were the first type of maintenance free batteries that came on the market, replacing the old wet acid type batteries first used on electric wheelchairs. No doubt, they were a great improvement over those dinosaurs but AGM is quietly taking over the mobility market.