"This thread has prompted me to think about how/why Wechat and Alipay managed to achieve dominant positions wrt payment."
I think it comes down to money. Using those systems, a merchant pays a tiny fraction of one percent to get payments into his bank account. Using Chinese bank cards, the fees run well over one percent, and there's the need for a special terminal and phone line. With Wechat and Alipay, a small merchant can just use his phone. That is, Wechat and Alipay have spread so quickly because the transaction and entry costs are practically nil.
Compare this with the fees of Visa/MC in the West. They have a lock on electronic payments in many countries, and aren't about to expand their business by cutting their very lucrative fees to Alipay/Wechat levels.