And, what is that reason?
I would guess that if there was a real business opportunity for a new operating system, somebody would jump in both feet waving. Some have tried in the past, of course. The reason they failed was usually because their product wasn't compelling enough for the market to gather enough investment, so people didn't grab onto that system. But, even IBM gave up and they certainly had enough resources that they ultimately chose not to deploy.
For most people, I bet their calculation on which operating system to use is based around both practical considerations (Will it run Excel? Word? Does it work with my school system?) and various intangibles, like how much the product sells for, what their personal acquaintance circle uses, and the effect of advertising and peer pressure. Adding more choices isn't always better for most people, other than in theory.