That's likely because you aren't in mainland China. Many of the commonly used chat applications in the US are banned in mainland China. For example, WhatsApp, Facebook Messenger, and Telegram are banned. Signal is not banned but it has very little usage among the population in mainland China, which is likely why it hasn't been banned. iMessage can be used but Apple controls the key management, which means the government can likely compel them to hand over keys for Chinese devices. Moreover, iPhones are much more prevalent in the US than in China. WeChat has a billion users - virtually everyone in China uses it - it's a chat, payment, and social media platform, and can even be used to pay your subway or cab fare in Shanghai. Moreover, for individuals with limited or no English proficiency, the options may be even further limited. The point is that chat software is only useful if the people you want to speak to are also using it. For communicating with people in China, there's no more prevalent app than WeChat. That doesn't mean it's not used for surveillance or propaganda. It's the only app I know of where your messages are censored in real time. Send a message about a controversial topic and it may never be delivered. You also may be interrogated the next time you enter China. It's a horrible app. Nonetheless, for many Chinese-Americans, it's essential for communicating with their family in Mainland China. Thankfully, a US-purchased device can use the US app store in China. US-purchased device + VPN to Tokyo = access to any software you wish to use.