The main 2 challenges I can think of is lack of native app support, phone hardware being pretty proprietary, and lack of carrier compatibility. Any truly open phone OS would have to emulate Android or iOS apps which would be very demanding on resources. Or just exclusively have web apps and fully open source apps of which there are very few for phones. Then you’d have to reverse engineer the chips that talk to the towers and/or get cell carriers to release firmware. Just overall a mess that has basically translated into the smartphone market being dominated by megacorps.