Simply because with 25 years experience with the nuances of the Windows OS versions, I don't have to wonder "what the hell?" and "why don't this work?"
I don't have to screw around with a new OS that has a VERTICAL learning curve, and it will not handle the applications that I know and use to maintain my business every day.... I have not changed my Word Macros in 20 years, they always work.... just a few keystrokes and whole paragraphs can appear if that is what I want.
I don't have to "tweek" Win7, I have it stripped to what is needed to make it shine. All of the junk was stripped out of it the 1st time I saw Win7..... it was nothing new to me, just like the previous version, a few new things under the hood, but nothing that changed the way I use the Desktop.... my Desktop has not changed in over 15 years... it don't need do. It works, and it works every day...
I won't bother with repeating the effort it took to make Linux work on my laptop, most of that was because of Microsoft's insistence on "Secure Boot", but once I figured that out, and turned it OFF, Linux installed, and I have now a Web Browser that works just fine. and for me, that is all a Laptop is good for, a Web Browser.... the OS under the hood I could care less about, as long as it works the First time, Every time... and for the laptop I have now, Linux boots faster, and runs Firefox just fine.... and that is all it will ever do for me... run a web browser.
when I want to do something important, it is always on my 24 inch Monitor/Desktop with gobs of RAM and a fast CPU plus I have a Professional, lighted keyboard, with keys that register a keystroke before the key hits the bottom of the stroke...
Laptop keyboards are a POS, I have never, ever, seen or felt a Laptop keyboard that is any damn good. and that includes the military versions that I have used from Panasonic in their toughbook series. I have been thru 5 versions of that laptop.
Linux has its' place, but not in my big desktop.