[gvb]

The primary reason is likely because the hardware (SoC peripherals) drivers were written for 2.6.x and not forward ported to newer versions of the linux kernel. A lot of hardware drivers were (are) written by the hardware (chip) manufacturers and then abandoned.

[throwaway2048]

when you say a lot, the reality is this is the case with basically every single ARM SoC on the market (and a few x86 ones too!).