Regulation by 3rd parties. People use phones for payments and accessing sensitive services. But these apps work only on Android / iOS. There those who don't seek this use case, but there are rather small and increasingly shrinking minority. This is compounded by the knowhow needed to install non-stock OS on phone in the first place.

SailfishOS, Ubuntu Touch, and Nura seem to be good contenders. One of them must become a real alternative to the duopoly.

This is increasingly becoming the case. There have been a few posts on Reddit of people successfully porting Arch and other distros to phone hardware (typically seems to be OnePlus?). My handheld game console, a Snapdragon 8 Gen2-based Android device by Retroid, just got official first-party support for running Linux on it.