The general theory that I have is that the Great Migration brought millions of chinese citizens to the city, who neither had bank accounts, credit cards, or phones. The pathway to connecting with friends, businesses, and money or even finding jobs immediately became "Get WeChat and a phone." This lead to a reenforcement cycle with businesses, that they had to offer services on WeChat to get new customers and combined with the injections of cash due to the overall modernization of China, it allowed WeChat to grow even more. And of course, add in the local government blocking foreign competitors.
Thus a large portion of some individuals where basically introduced to modern computing via phones and via chat and businesses started advertising "Chat-first" as well.
I've seen roughly the same in India with WhatsApp, but with a lot less discrete functionality.
Feel free to correct my theory though, it's mostly outside observation...