[aruanan]

with Windows 7 having already sold a staggering over 550 million in such a relatively short time.

Because it was a replacement for the incredibly balky Vista.

[SmokingJoe]

“Because it was a replacement for the incredibly balky Vista.”

Nope. Because Win 7 is a great OS.
The bulk of Win 7 sales were XP(not Win 7) replacements. Untill Win 7 shipped, XP still had by far the biggest desktop OS installed base.