One of it's main strengths probably was how easy it is to use (e.g. plenty of the competitors you can't just sign up for a free account and then invite people without an account) while working very well (which many of the WebRTC-based competitors in the "easy and free to use" category struggle(d) with). And then it quickly becomes self-reinforcing: if people get introduced to it they'll use it too unless they have a good reason to seek something else, which gave it a boost once video conferencing suddenly was something many people needed. (by now Microsoft Teams has anecdotally taken over lots of the corporate use, tech-y communities have gotten around to bring their own jitsi and BBB instances up, ...)