Momentum, support and compatibility.

There are also other OS’es like FreeBSD and openBSD that are relatively widely used and a whole host of vendor OSes like IBM’s IAX or Z/OS or the open solaris derivative illumos (all unix based), not to mention the embedded real time OSes that you find in a lot of cameras and such.

The common thing among most still in use is that they are old, well tested, stable, have a lot of software developed for them + they are in most cases compatible with a lot of different hardware, these things need time and money to achieve and people aren’t going to develop software for an OS that isn’t going to be used because it lacks those features.

That’s not to say people aren’t still writing new operating systems, they definitely are, it’s just that they’ll never get as generally used or well known as the mentioned 3.