There are a few reasons. Its not very polished, one screen is classic windows small font, the next screen sometimes even in the same app is the large baby font touch screen interface. It's extremely inconsistent, switching between visual contexts that are glaringly different.
Microsoft have broken the right click context rule. Right click your current wireless connection and you get nothing. You have to left click it. Yet other UI elements are right clickable.
A lot of shit breaks, especially printers. Holy crap adding printers to Windows 10 is a nightmare. Windows 10 tries to be intelligent and ignores your suggestions for drivers and names. Set up a new printer, and you give it a custom name, it comes as something different. This is a bit issue with WSD devices.
The printing subsystem is broken as hell starting in 8 and still a problem. An example is one company i do work for has "department IDs" for tracking printing.
Print from an office program you are fine. Print from edge or any other native windows 10 app, and windows totally ignores the department ID, and gives you no way to add it. It simply doesn't print at all. No error. Just wont print.
Removal of safe mode?? you cant press F8 to change boot options like get into safe mode? What the fuck of all things would they remove that?
Lack of choices. At lot of screens just have "ok" on them. You cant cancel, you cant close, you will DO what microsoft says you will DO, or you stop work. Its that simple. Windows is.. was a multitasking operating system, yet your entire workflow comes to a halt because of something like a windows update, "microsoft is adding features" or a variety of other un-admissible bullshit that just stops you using your computer.
Lets break things. Lets totally break the guest account, so you can no longer share files or your printers. What are you going to do, go to a different OS? hah. its microsoft or broke for 90 percent of the world.
And that comes to the crux of the situation. Its no longer your computer. You have to fight to stop ads, suggestions, get your privacy back, you have no choice on which updates and when to install them, or to cancel them because you are running something that absolutely cant be rebooted at 3am, much less all those open files you had with critical information that are now gone because microsoft released a patch that decided to break outlook. (this happened 2 days ago)
Its not your PC anymore. You have no idea what information microsoft is gleaning from you, what they are using it for, what income, what privacy or rights they have to your information because you clicked yes on everything just so you can get back to fucking work so you can pay rent and buy food.
No. Lets install windows 10 with a popup box that treats being closed as ACCEPTANCE after teaching users for over 20 years that clicking close will close the program.
Lets take the goodwill, the muscle memory, the training, the ability to do your job and throw it out the window for a clunky hacked up GUI that any designer would scoff at, while laughing that the irony that microsoft breaks their own GUI design rules at every single turn, then waiting to continue working because the OS decided to reboot while you were working.
I would rather use Windows Vista then 10. Hell windows 8 itself was semi decent with classic shell.
Windows 10 is an abomination of glaring design, privacy and usability issues.