the main hurdles from my understanding are processor/chip specs. They are generally super locked down in terms of who they run with/allow usage. From what I understand the liberux project ran into that issue because their goal was a fully open sourced Linux phone, and they had to make compromises and are still fighting issues.

my guess it's hardware support; drivers and firmware for existing hardware and smartphone components for linux are probably close to nonexistent. The apk support is the smallest issue.

The main mobile OSs country (USA) and the main mobile builders country (China) don't like "free range mobiles".