Sometimes they can afford to not know because they have a relative who can manage the device for them. In which case they really wouldn't be installing software on their own really.
You would be surprised at how many people cannot even change the volume on their tablet.
Popularity makes all forms of support infinitely easier. I'd struggle to come up with any technical reason that could be worth giving up the ability to easily google for issues or install software. That doesn't mean I think you shouldn't use other distros, just that I believe Ubuntu is the best choice for a default install targeting average people.