No, its because the NSA backdoors it.
It's also NOT the most used.
GNU/Linux is the most used OS in the world if I'm not mistaken?
You're grossly mistaken.
http://en.wikipedia.org/wiki/Usage_share_of_operating_systems
I humbly take it back
I was under the impression that billions of servers were all using linux, dwarfing the desktop PCs in numbers. And then there's android which is huge (and also linux)
first one would need to define what it means to be "most used"; one could argue that Windows is the most used, if one defines used as used like the bathroom stalls at old inner city greyhound bus stations