There are others. Firefox OS for mobile, Ubuntu, and a few others I know of but cannot recall off the top of my head. Sony-Ericcson had one that pre-dated iOS and Android, and of course Blackberry has one too. All are, afaik, dead or defunct or dying on the vine.
The main problem with the older ones is that they are out of date. The problem with the newer ones is that they are open source. The problem with open-source is lack of profit motive for a developer and a lack of stability for the user.
Users want to know that it will work and that all their apps will work. There are other larger interests at play - government doesn’t want an open source OS to take off. Linux failed; and only succeeded when Google made their proprietary version of Android (which they bought and then improved). Linux itself flopped as a consumer product and really only took off when someone (ie. Red Hat) customized it, branded it, and then went on to build and enable others to build specialized apps for small market niches where they could make a profit.
The reality is anyone can get an open source version of Android and put it on a device. The problem is it won’t be perfectly compatible with all the apps, may not work with all devices or bluetooth, etc. With open source, no profit incentive, so no incentive for long term development.
Windows 10 might be 3rd place behind Android and iOS, or maybe 4th place but it’s so far behind I am hard pressed to think of who would be 3rd or 4th. Blackberry switched to a proprietary version of Android that runs most Android apps but they are sucking wind.
I wish to be freed from the walled gardens of Google and Apple but we’ll have to wait. I’m very unlikely to ever buy Hauwei’s OS.
[The reality is anyone can get an open source version of Android and put it on a device. The problem is it won’t be perfectly compatible with all the apps, may not work with all devices or bluetooth, etc. With open source, no profit incentive, so no incentive for long term development.]