Why Windows 8 we have barely used 7?
1985: Windows 1.0
1987: Windows 2.0
1990: Windows 3.0
(1992: Windows 3.1)
1995: Windows 95
1998: Windows 98
(2000: Windows ME)
2001: Windows XP
2006/2007: Windows Vista
2009: Windows 7
2012: Windows 8
Nothing out of the ordinary.So from a technical standpoint, Vista took longer [5 years] and had less development time [under 2 years] actually make it into the product. (Yes, code they already had they could and did reincorporate into the final product, but still, wow.)
[1] http://blogs.msdn.com/b/michkap/archive/2005/10/16/481625.as...
This post is full of snark by the way, but it is useful to know what went wrong with Vista. (TL;DR: Management of very large software development is _hard_)