Safari increasingly doesn't follow standards, implements non-standard features, and updates are tied to OS updates so if users do not update their OS (which is free on macOS but not well advertised in the OS itself), they are stuck running an old browser.
Huh. That's changed, then, since previous updates were always tied to the OS. Still not great support, though. Chrome supports back to 10.10, Firefox back to 10.9, Vivaldi back to 10.10, Brave back to 10.10, etc.
There are multiple articles online from people claiming that Safari is the new Internet Explorer (IE) in terms of feature parity with other web browsers like Mozilla Firefox and Google Chrome, and specially due to the fact that Apple usually delays the release of certain HTML and JavaScript standards, making web development slightly more difficult for today’s web developers.
At least on IOS you're stuck with safari/the system webview, so even if you make a web browser app (this includes chrome/firefox) they're using the safari engine.
The issue stems from the slow adoption of certain technologies. That was the issue everyone had with IE 6 - everyone used it but it lacked modern features so it sucked to use, but you had to because of it's market share.