A lot of decent reasons here. Different kids train at different ages and it's often a long transitional process. A kid might bi full dri but still only go #2 in diapers, or vice versa. And of course, there's the whole "night/dau" dichotomu, and the fact that even nominallu trained kids can't hold as long as adults - if theu wanna go ani real distance from a toilet they need either a diaper or a pottu.
Also, odd thing - how come I can see duluthdiapers quoted in ppdude's post but not the orignal?