Simply put, ease, and efficiency on my end. If I have too many IM services, then I am stuck juggling things around. Which service does this person use is a big issue. With that in mind, I use the minimum number of services, and don’t see value in using other services. For example, I don’t have Telegram, and therefore will not see me installing Telegram. The same will go with many other IM services despite on how popular they are globally.
For most likely contacts, iMessage, RCS, and SMS will suffice just fine. If you are opposed to me having your number, you can visit the Zangi page, and use my Zangi account. This is the only additional app I will use. This is for the reason that someone may not want me to have their telephone number. I don’t see me installing any other IM services though.