Native Linux on Android's SOCs

The main reason:

There is no revenue-generating stream for Linux on smartphones in comparison to Android.

Technical reasons, which can't be solved without heavy investments

Just porting Linux to SoC is not enough. Linux is only a kernel.

• You will need to develop a distro suitable for a smartphone.

  
  



• Android = 1. AOSP + 2. patches and blobs from SoC vendor + 3. user apps`

  
  

Patches and blobs from SoC vendor

#2 in the equation has restricted access. SoC vendors don't share internal details even with OEMs, only providing a minimum description for available APIs. Typical Android-capable SoC has a lot of proprietary peripheral modules (such as mentioned GPU or Audio, Image processing, radio modules). Some of them require OS drivers, others even specific complex FW with a separate OS.
Even for some desktop peripherals, this factor blocks Linux support.

Existing attempts

There were attempts to develop an open-source smartphone. As we can see, they didn't get widespread till the moment, and some of the projects were discontinued. The main reason is the same: the model didn't prove its sustainability.