The OS needs to support X device the person is using
People need to know how to flash the OS(Graphene has made this SO easy, but still)
People have to want to flash a different OS
I remember flashing Android on my HTC HD2 20 years ago. I'm a super geek for this stuff. I've been wanting to flash Graphene on my pixel but...I need my phone every day. I can't have a phone out of commission to try and find a way to install all the apps I need. With Graphene I can resort to just installing the Play Store but...I'd rather not use the Play Store at all if I'm already de-Google'ing.
I'd love to have a Linux phone, not android, but I haven't seen a solid alternative yet. Correct me if I'm wrong.