Call quality was not the factor. It was that zoom was easy for people to use, no download required, just click a link and enter your name and you’re connected. No logins or downloads so old people could actually work with it.
I never used the zoom web version, but for the client: it simply worked. Getting videocalls with WebEx always was a pain, where it was noisy, video hat issues ... switching to Zoom was day and night. Suddenly doing a video call was a click and it worked.