Isn't WeChat closely monitored by the Chinese government? I'm not sure why anyone outside of China would want to use it (seems like volunteering to be wiretapped).
At least in the US or other western countries there some legal restrictions on the government for accessing private messages.
communication with friends or family in china, mostly. when most common western social media has no foothold in china - or is simply blocked - getting a wechat account is the simplest way to stay in contact.
and i'd imagine most of those people don't care much if the chinese government is spying on their baby photos or vacation albums.
At least in the US or other western countries there some legal restrictions on the government for accessing private messages.
In China there's probably not much of a choice.