There were alternatives... But Skype was getting worse by the day, moving from decentralized to MS servers for all calls, and support for everything besides MS and the official App was also non-existant.
Google Calls/Hangouts/Meet/? was mistrusted by all MS shops (and still is)
Teams was nowhere it is right now beginning of the pandemic, and required buying into the Ecosystem.
In fact, Zoom was a (!) low entry, mostly platform-independent solution that was easy to set up if you had nothing to start from. And joining a call was not asking much from customers, students, business partners, etc.
All the competitors learned a lot from Zoom and we take these things for granted now.
There were other solutions of course, but sometimes you're lucky, and ads help.