One of the factors for Rest of the world vs China gap could be the ticket size of a cab ride. A 5km Uber ride in India/China/SE-Asia costs $1.5 on an average, while in developed countries its $15 on an average (10X). So, I guess, it's value over volume for Uber and markets that are on fast-track to profitability.
source - http://www.priceoftravel.com/6536/price-of-a-5-kilometer-ube...