Why? Well unlike (say) GoToMeeting or Webex, you can use it for free (albeit with a time limit). Until recently it had a much more modern, easy-to-use interface than GoToMeeting as well. Also, apparently it's one of the few videoconferencing solutions that works reasonably well in China.
Also, it apparently scales pretty well; one of the groups in my church apparently had nearly 100 people in a zoom.us conference last week. I didn't participate in that one, but there was a distinct lack of "and that was a disaster" comments.
I resisted using zoom for about two weeks, specifically due to the "start a local web browser to work around Safari's security features" disaster; but ultimately, you need to say "no" to every other person who wants to have a meeting with zoom, and eventually I just had to give up and install the client.
(I've been recommending meet.jit.si since it's 1) open-source 2) unlimited time for the free version 3) doesn't need to have a client installed.)
Not too much it's easy to use. Everyone, from boomers to tech-challenged zoomers, can easily sign in and use it.
Probably it spreads from company to company as people organized video conferences with external companies, introducing the tool to those people, who decide to try it out, etc.
My biggest complaint is that, for new users, it seems to choose either the wrong speakers or the wrong microphone about 80% of the time. Once that's fixed, it mostly works well. The interface is a bit clunky, but I can usually get it to do what I want without too much trouble.
Source: I've been a mostly happy user of Zoom at a university for two years.
As far as I know other competitors would be Skype (Microsoft) and WebEx, neither of which seemed nearly as polished.
Wiggum: The McWhat?
Lou: Uh, the McDonald's restaurant. I've never heard of it either, but they have over 2,000 locations in this state alone.
Eddie: Must've sprung up overnight