Adding payment processing to a chat app is just really, really convenient. I'm Spanish and we have a thing called "Bizum" here, you can send and receive money from and to anyone in your contact list instantly and free of charge. It's hugely popular even with no chat implementations.
Venmo tries to push it's messaging features but nobody really cared. People use Venmo because it's free transfers, they don't need a new chatting app that mixes money with memes. Welathsimple Cash tried that here in Canada but hasn't been terribly succesful either, and so has WhatsApp in India.
The reason WeChat become popular because it had first movers advantage and used it's reach to be the first payment service of it's kind. People didn't use it because it integrated with chat, they used it because it was good.