Popularity =/= quality.
That being said, obviously the Librem phone is missing functionality.
Most computer users are goo goo ga ga level users. Most people in the US use iMessage, for example.
It's not the best messenger, not even close. It's not the most ubiquitous either - doesn't even break top 5 globally. Doesn't have the most features. Not the fastest. Not the most secure. Not the easiest to use.
They just use it because it's already installed and right there. That is how deep their understanding and comprehension can go.