The kernel you get is entirely up to the SoC vendor.

Snapdragon 820/821 ships with 3.18. Snapdragon 830 ships with 4.4. Snapdragon 830+1 will ship with 4.9

There's a huge amount of work involved in porting a chipset from one kernel version to another. While I'm sure Google _could_ do it, it's ultimately not worth their while - they'd probably lose support from Qualcomm, and if they hit any weird bugs they'd be on their own.