Because, for years, Chrome and its forks were basically the only good browsers out there. Both Internet Explorer and Firefox were in bad shape at the time.

Internet Explorer never recovered its reputation (and Edge itself is a fork of Chromium, anyway), and, by the time Firefox recovered, too many people were integrated into the Chrome ecosystem, and too many of those people believe it'd be too difficult to switch (and most don't really care since Chrome's issues have yet to personally affect them).