Part of it is that, (at least in my experience, maybe this has changed) Zoom can scale to a couple hundred people in a way that tools like Google Hangouts can't.

But like, if we're being honest, it probably has a lot to do with how easy it is to start a Zoom call and invite people. You can host a 40 minute meeting for free. No one needs to sign up anywhere. It's super easy to install but also works in the browser if you can't install it. Computer on the fritz? You can call in from your phone. And yeah, they've also used some dirty tricks to make it as easy as possible, and some of those measures (like the auto-reinstall thing) were probably unnecessary. But they've clearly focused on being super super easy to start using, and when their moment came they were primed to seize it.

Last weekend my family had a "month's mind mass" in memory of my grandfather who passed a month ago. We were able to get dozens of people, many of them very non-technical, into the call, and we started basically on time. There was no "it doesn't work on my old phone" or "you mean I have to sign up for gmail?" or "whoops I couldn't get in because I signed in with my work email". That's why Zoom is winning the game right now.