I think it's mostly still sold because of that retail environment thing. They're cheapish and easy for businesses to deploy - mainly in my experience for wait staff at fast casual restaurants. I assume that selling them in extra capacities and colours isn't a big deal (although the fact they do suggests there is a consumer market for them too).
You could argue "why don't they use an Android phone" or "why not use an enterprise thing from Zebra" - I suspect the answer to the former is that there aren't any that have the same small/light/good/cheap/durable ratio, and the enterprise tools Apple provide are helpful, as are anti-theft measures. The answer to the latter is a handheld from Zebra costs about six times as much and is 5 times the volume.