Backwards compatibility was important on the web because there were other browsers to compete so each browser had to prioritize.

If one browser broke backward compatibility in a specific way, that could lead to a site not working in the user's browser, but still working in the other browser that hadn't yet broken it.

Users can switch between browsers (relatively) easily, so sites need to keep working so users don't leave.

That forced the web to have an emergent, inescapable policy of backwards compatibility.