Sure, the network effect is (an important) part of it.
Another - in my opinion not less important - aspect is peoples persistence with something they are used to (most likely out of laziness).
When buying a first phone, the network effect might be most effective, as almost everyone you are asking is recommending either iOS or Android. Prior to this forum I never ever “met” someone using another OS. If you are not really doing some research, you will have a hard time stumbling over Sailfish or the like.
And once you are accustomed to Android or iOS and have learned all the tricks how to make the most of it, it usually takes some really good reason to make you change.
My guess is, that such a reason is hard to find, when it comes to functionality, as Android and iOS encompass years of developement and tuning.
Data protection/privacy can be a reason; although it’s not for too many people, as social media usage, the fate of Blackberry and many more things are proof of.
Frustration or negative experience with a device or the OS might be a reason as well (but hardly so on a large scale).
Factors like environment, fairness etc. are good reasons as well.
In my opinion Fairphone clearly shows this problem. Although it is more easy and even advertised to switch the OS on the FP2 - at least ot FPOOS - it’s a minority of users doing so (myself included). Why?
It’s “work” without any functional gain in the end (if you are not a geek in need of a rooted phone or functions like twrp etc.). There’s always the fear of losing data, malfunctions and trouble in general.
The network effect in this regard - at least for me - is of minor or no importance, as all apps would stay the same.
Well, I tried to switch to Ubuntu Touch but I failed, no matter how hard I tried. In the end I decided to stick to FPOS and maybe give it another try later on. I know, it’s totally nonsensical, but that one (multiple) failed attempt in switching the OS has done it for me for the time being. Right now, I don’t feel any urge to go for FPOOS, Lineage or any other OS.