Fundamentally, a wechat/alipay transaction is traceable. Traceability is great for expense reports, but a non-starter if you're a drug dealer -- especially with China's draconian drug laws.
Even if you were to launder the money through fronts or a series of transactions, a state level actor (i.e, China) would have no real difficulty in tracking down such activity. With cash, this becomes much, much harder in cost, time and manpower. Not impossible for a state, but certainly an effective deterrent from casual or low level investigation.
This difference in traceability will keep cash and other compact tangible mediums of value (precious gems and metals, ivory, drugs, etc) in use by subsets of the population for as long as people value the relative anonymity and are willing to put up with the costs and risks a physical medium entails.