Comment by c2h5oh

Qualcomm doesn't bother to upstream most of their SoCs. They maintain a fork of a specific Linux kernel version for a while and when they stop updating it or new version of Android requires newer kernel then updates for all devices based on that SoC end.

They have little experience producing code that is high enough quality it would be accepted into Linux kernel. They have even less experience maintaining it for an extended period of time.