This is speculation based on external observation, nothing internal other than rumours:

A big, increasing over last decade, chunk of that is fear that they will break the compatibility - or otherwise drop in shared knowledge. To the point that the more critical parts the less anyone wants to touch them (heard that ntfs.sys is essentially untouchable these days, for example).

And various rules that used to be sacrosanct are no longer followed, like the "main" branch of Windows source repository having to always build cleanly every night (fun thing - Microsoft is one of the origins of nightly builds as a practice)