The simple answer to "How" is simply that Win 7 (or Vista or XP, etc.) is the windows that everybody has learned, grown up with and knows how to use. And it works, very well! Could there be something better? Sure, but Win 8 definitely isn't it.
All the platitudes fit very well here, #1 being "If it ain't broke don't fix it". Why on earth would anyone want to learn a new system from scratch after 15 years or so of using something that always worked fairly well and has gotten much, much better? There is nothing in Win 8 that jumps out at people as being any great improvement, and the negatives are apparent as soon as you turn on the computer. As to faster boot times, Win 7 is plenty fast enough, and if you need speed an SSD C drive will give a lot more improvement than Win 8.
And I'm frankly, quite sick of all the gibberish from people saying install this, that or the other program and it will make Win 8 "almost" as good as Win 7. That's both absurd and insane. Just as it's nuts to tout all the advantages of Win 8 when there is not one feature that could not have been built into an upgrade of Win 7. I've yet to see the first person say they just can't live without a touch screen on their desktop or office computer. If MS needs a different OS for the touch screen feature, then fine, use Win 8. Just dont foist it off on the rest of us.
If enough people abort the Win 8 monstrosity, maybe, just maybe, MS will come to it's senses and make Win 9 a version that people can recognize again. If not I'll be an Apple guy, much as the words catch in my throat.