A lot of the split-pants thing can be explained by one phenomenon: grannies taking care of the kids. The older generation still believes that this is the best way to deal with non-housebroken kids, even in households that have enough money to fund diapers, plastic pants, etc. Moms that go back to work (= most Chinese mothers of kids older than 6 months) typically leave their baby or toddler in the care of the nainai/ laolao (grandma on father/mother side), and pretty quickly learn that tangling with grandma's methods on matters of early child care is an exercise in futility.

I see a lot more diapers and little or no split-pants on little kids whose mothers are the primary daily caregiver due to not needing to work. Urban moms (usually under age 35) have a more "western" approach including using either cotton or disposable diapers. In smaller cities and rural areas, split-pants rule. You have to admit that the diaper-rash problem is greatly lessened with the traditional method.