Some other points you made aren't the main drivers of success:
People pay for everything with cash They also have bank cards and Alipay, and had them before WeChat payments existed.
Sometimes you don't bring enough cash with you Yeah, and I used to use Alipay to reimburse friends, before WeChat payments.
Paying for each others' meals WeChat doesn't help with that. It helps you pay for your own portion, as you can use WeChat to reimburse the person who paid the restaurant.
People don't have credit cards I'm not sure that _credit_ is relevant. Everyone has a chip+pin bank card (what I'd call a debit card in the UK). People have been paying with bank cards in restaurants for many years.
You made one point which is important: "Everyone and their mothers has a WeChat (I've never met anyone that doesn't... it's more ubiquitous than Facebook in the US)"
But practically everyone has an Alipay account as well. The thing is, I have 100s of contacts on WeChat (which I added for messaging) but only 10-15 on Alipay. It's the existing social graph, not just the number of users, that's important.