Most people don't want to trade off the size and weight for the flexibility of batteries that they'd rarely, if ever, take advantage of. I have a secondary bike light that takes AA's, it's both much larger and much less bright than my main light, which uses a LiIon battery. I haven't found a comparable AA powered light to my LiIon light. I got the AA light just so I could replace the batteries with easy to find AA's just in case all of my batteries ran low...In the 5 years since I got that light, I've never needed to replace the AA's.

Since most devices are rechargeable via USB these days, most people are probably better off carrying a USB battery pack that uses AA's, then you're only paying for the bulk once (and it's quite light without batteries)