It's basically because of drivers for the SoC (system-on-chip, the integrated chip from e.g. Qualcomm that has cpus, graphics, sound, camera, power management,

It's basically because of drivers for the SoC (system-on-chip, the integrated chip from e.g. Qualcomm that has cpus, graphics, sound, camera, power management, etc). They're too messy/hacky/low-quality to be "upstreamed", it is mostly impossible for anyone but the SoC vendor to port to newer versions (closed-source binary blobs), and the SoC vendor is really not motivated to do so.