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 contacts, I will use the IM services provided in this site. This is to keep things simple. I will not install any new IM service or clients though. As mentioned in this FAQ, I don’t want to have to juggle through multiple platforms because everyone is a little different.