A variety of answers, including but not limited to:
I don’t want to learn another new thing.
There aren’t enough other people I regularly contact using it.
I already have this thing, why add another?
I don’t care about the benefits of switching so why would I?
I like what I have.
Again, I can understand these reasons. (Especially universal adoption; it’s hard to get someone to switch to a messenger that almost nobody they talk to is using, just so they can talk to you.) But I’ve made my choice for a variety of reasons to which I wholeheartedly subscribe and don’t want to go back, but it’s had the effect of isolating me.–