Even if NT did provide slight speed advantages, which makes users happy and therefore the admins happy, the extra cost of NT is not justified. Microsoft is trying to copy the better operating systems, while UNIX forges ahead. Microsoft makes what few things you can do with your system fairly easy. However, if you go beyond what Microsoft has decided to implement, either you can't or you pay major bucks. Microsoft is submission to what Microsoft thinks is best for you.
Money is power.
Bill Gates and Microsoft have a lot of money.