I don’t really understand how its market share is so low now.

Everyone has a Google account, Chrome comes preinstalled on many web-enabled devices, people don't realise how bad Chrome is compared to alternatives, people don't understand they can search with Google on any web browser, etc. Most people are not particularly tech literate and don't really understand what they are doing. They just use the most popular/advertised product and assume it is the best choice for them. Even in Lemmy privacy communities, where you'd expect users to be more tech literate, I've come across many people who don't even know that browser export/import is a standard feature everywhere, or that other browsers have their own versions of cross device sync. They think they're locked into Chrome and moving to Firefox would mean completely starting again from nothing.