China is building whole new cities, and is still rapidly expanding the existing ones. It’s much easier to start from scratch with the latest technology when you don’t always have a big sunk investment in an older system.
Yep, it's the same reason many developing nations don't have any serious landline telephone infrastructure, but everyone has a smartphone. They didn't have landlines before, so as they got richer at the same time cellphones came along, they just skipped over the older technology and went straight to the new one.
They do have a much more serious air quality problem to deal with.