Why is there no other brand except iPhone Apple that has its own operating system?

And there are many more open source OS.....just not famous enough to compete with the giants

Sorry but there's a list of smartphones that have got there own operating system.

Google Devices(eg. Google pixel, Nexus 6p,5x,5)have Android

Microsoft phones have Windows CE

Blackberry has Blackberry 10.

Firefox OS is also out there from mozilla.It’s an open source mobile operating system released under the Mozilla Public License built on the Android Linux kernel and used Android drivers, but did not use any Java-like code of Android.

Samsung is working over it's own os called Tizen OS.

For establishment of a whole operating system, it needs millions of line code, any one man suppose to be not adequate for operating system. A serious team work is needed. apart from that, marketing is the main concern for individuals/ Non profit organizations. Corporate big houses like Apple, Google, Windows facilitated a rich background market for releasing of operating system, it is very obvious, OS could be very popular. Though there might be some debate that, the quality of OS also very good and that turns people towards it. But everyone should agreed in the fact that, if background market was nowhere, then it would be quite tough. Don’t think the situation of today, rather the first day when released the OS.

On the other hand, Mozilla released firefox OS, Sailfish, Tizen and so on are not very popular. The probable reason is appstore. Android and apple has made their appstore so resourceful, that anyone new in the market could lose before they start.

Apart from that, it is very evident true means of inventory is missing, only old wines in new bottle are the only trick today to fool the customers.There are many other companies too like black berry but the optimisation on Apple it so good that , the fact it that it's optimisation is better than Android so Apple has managed to put low power cpu and get better performance than quolcoms Snapdragon 835 latest from Qualcomm and sure the software experience is much cleaner on Apple. Making an os by an new company is hard as they have to invest a lot on research which is difficult for a new company. So many companies tend to adopt Android.