@johnny Just speaking in the capacity of a company operating within Asia.
WhatsApp has been the instant messenger that achieved traction in most SouthEast Asia markets in the early 2000s after other messengers like LINE and Viber failed.
LINE then focused on certain markets like South Korea, Thailand and Taiwan with great results due to their early and mass usage of Emojis and they are the de-facto messengers in those countries to this date. Interestingly these countries also like to use Facebook Messenger as their instant messenger.
Viber was the only messenger that could reliably send messages and calls through the Myanmar restricted internet, and the large number of Burmese people working overseas use it to call family.
The black horse messenger app that has no region limit is WeChat, the most intrusive spying messenger that anyone who does business with China, have no choice but to use it. Some China suppliers have been adopting WhatsApp more and more these 5 years due to meeting their client needs.
Telegram is another black horse which gained rapid traction in the past 5 years within Asia as it was used by the younger generation for the public channels and deletable chats.
Signal is just not installed on a large enough consumer base in these regions and thus no one bothers about it.