Sure, network effects are hard to overcome; my point is that Zuckerberg bought it as a competition killing move so they're not likely to put any quality control into it
I guess you are entitled to your own opinion (which I disagree with, though I recommend using Signal because the clients are open source).
IMO the answer to your original question is: lock-in effect. People are generally too lazy to move to an alternative, and on top of that, if they decide moving to one, they don't necessarily know which one to chose. And many privacy-advocates don't help by being (again IMO) too extreme: "Do NOT use Signal, you MUST use <choose your complicated ultra-secure alternative here>".
