Internet Explorer (formerly Microsoft Internet Explorer and Windows Internet Explorer, commonly referred to as Explorer and abbreviated IE or MSIE was included in Windows since 1995. That is 24 years now and from day one it has had issues with compliance or keeping updated to standards of the time. It had features that only it supported at different times as I assume the team at the time thought the "standards" were behind. It really doesn't matter as the browser is very important today for every user regardless of platform they're on. Microsoft has more dedicated developers than any other software vendor on the planet and some amazingly smart and talented Engineers.
The real issue I would venture to say is that top level management in the past 24 years cannot be convinced to allocate adequate time, resources, and money to put an end to what I think most probably feel is an embarrassment for Microsoft that they have not been able to be the leader in this area after even 24 years of being at it.
I could be wrong but I would bet that most people don't really care what the underlying technology used is. They just want a browser that works with every applications that needs or calls that browser on Windows 10. If Chrome, Firefox, Opera, etc. can open or work with it and decode the request properly then Microsoft needs a single browser that can do it and replace all other browsers. Then as the technology changes, Microsoft needs a framework that this new browser can be updated too. Microsoft has made fantastic advances in cross-platform usability for applications etc. - look at Windows Defender. On XP, and Windows 7 it was a joke, on Windows 10 it's actually much closer to just about any other full antivirus suite out there in terms of protecting the computer. Stop ignoring the browser Microsoft and build a World class browser already, I know they can do it if given the chance and support from top management.