The school district ended up distributing their own Gmail credentials to use for the meetings, too, which was behind a hard-to-find edu-website login. That's a lot of friction that Zoom solves.
In March, when everyone suddenly wanted to do group video calls with their friends and family, most of whom are nontechnical, we settled quickly on Zoom for ease-of-access. Of all the options, it was the easiest one for me to get my parents into a family call.
You message someone a link, and they click the big buttons until they are in a call. No account, no confusion. We had 100% success rate on getting people in.
Also, it's asynchronous unlike something like FB messenger or Facetime (ie it doesn't push you the meeting, you can pull it at your convenience).
I used to use WebEx on a personal laptop for work conversations with other organisations because my work machine wouldn't allow it to run properly.
That and the fact Zoom had seamless integration with large meetings with relatively little-to-no performance/quality costs is a huge benefit. The luck of timing & advertising is what pulled it all together.