Because even though the list of 'improvements' is a long one, it is still a dog for people to use because it takes the people component and slows it down. Windows 7 is very well designed and very functional, so why MS decided to move away from the Windows concept and move to the Metro interface (non-Windows) is a mystery. The result is that there is no Windows anymore. Touch screen is cool, but I have that on Kindle and do not want it on my PC especially if that means losing the efficiency of the Windows 7 interface. Windows 8 is not efficient, except on the computing end and even then it does not save enough to make it worthwhile (and Skuzzy points out Windows 7 can be streamlined to work faster). Also, you can make Windows 7 use a touch screen if you really want to.
The whole reason there is a Windows 8 is because MS wants to make Windows a consumption vector by which they get more of your money. Why else would MS, a company known for selling their premiere OS, offer Windows 8 for a mere $40? The answer is simple. Windows 8 will open your wallet to them. The only really problem there is the Windows 8 app store is not a well stocked market at the moment. That will change.