Why do you think windows is The most used os?
Do you think linux like UBUNTU OS is hard to use for a non-expert users?
Windows is more user friendly and the the oldest one after the Macintosh OS.but also Mac developed nowadays and also used in apple devices like ipod , ipad and iphone.
Linux is newer one and it's platform build for developer interface and users.
Windows has both option of normal users and developers and I think that It has the most user friendly platform.
The reason for that is low prices and variety. Is the same reason android is the most used os on mobile.
My personal opinion is mac os and ios is the most user friendly to use but the high prices and even more now they raise ever products killing any idea of iOS or mac being in any given time the most used OS. But that also goes allong with there marketing strategy they don't want to be in every pocket.. with Tim Cook I think the idea ( and wrong one) they want to be in few pockets but those who pay more.
I think windows being much more affordable and having many OEM helps alot for any person to have a computer and that is great. Also the pre-installed OS on computers is what make people have a windows computer. Linux you have to do the extra mile to install it and configure, update it is not for everyone that only wants a computer that 'just work' out of the box.
Ubuntu is a good OS and light one improving everyday, but is always target for people that have some extra knowlege base. Gaming using wine... updating by terminal sudo package.. there are a lot they could do to make those steps easier and maybe one day be a plausible competitor that a common person start to buy products with linux pre-install and have a good experience without thinking a lot about every task.
Still in Desktop, Microsoft Windows is the easiest one when compared to Linux or Unix.
In mobile its a different story - Windows based OS is almost out of the picture.