Microsoft isn't Apple. Their entire business relies on retaining as much backwards compatibility as possible
Every time they make any sort of major, sweeping change. They get raked over coals until they're charred to a god damn crisp
When Windows went from the old DOS-ish 95/98/ME to the much more stable NT based 2000 and XP, lots of things broke.
XP only survived the Microsoft EOL chopping block because of the rise of Netbooks. And even then, companies pay millions to keep that afloat due to how entrenched it is (Something likely to be repeated with Windows 7)
Vista brought a ton of (arguably much needed) changes. But its hardware requirements were pretty high for the time. Along with buggy drivers from Nvidia, that sank the entire operating system. Both in public perception and market share
Windows 7 (on release) was arguably a UI-tweaked Vista. Hardware had caught up by then, and a lot of people agreed that this was the "good" upgrade from XP
Windows 8 came out and tried to shoehorn that whole "Metro" interface and converge everything around it. And it flopped almost as hard as Vista did
Windows 10 Microsoft played a much stronger hand and dragged everyone they could (willing or otherwise, if you believed some accounts) over with a "free" upgrade for most. But it also pruned back Metro pretty hard. Instead trying to chase after the "App Store" model