So on the older operating systems, programs that shipped with the OS will be embedding the browser as a COM component, usually for help purposes. If they're rendering local HTML (or generated HTML!) then they require that to keep working. So that particular interface has to be maintained, making it very difficult to upgrade.
Microsoft were kind of correct in the "browser choice" lawsuit that they'd used the browser as an OS component, and everyone else was correct that this was kind of a bad idea.
On the other hand, how do you embed a browser component in a forward-compatible way?