There are many different apps to choose from. Some of them allow the users to post pictures, others allow the users to communicate by text or phone and others allow money to be exchanged.
However, WeChat is the only one that allows all of those. That is one reason why WeChat is so popular. WeChat was created in China which is why it is also popular. Many of the apps take off and become successful in the country where it originated.
Then if it is popular enough, it can move and become successful in other countries. WeChat became popular first in China and it is really only used in Asia. In the western world, very few people have WeChat unless they are communicating with people frequently in Asia.