What vesak has said is partly correct. We are integrating IMs that have the most reach firstly.

We also investigated IMs where users are not registered to a particular number and we deduced that most companies would not want clients to authenticate themselves with accounts that are not attached to a number (when they already have such numbers available).

We stand to be corrected and would gladly integrate with other IMs where demand/interest exists.

I'm not sure about their reasons, but I would guess it's because the three messenger they support (Telegram, WhatsApp, WeChat) are significantly easier to integrate with, and have nearly a billion active users combined.