why is the r9000 firmware using linux kernel version 3.10.20

The current supported linux kernel version is 5.9.8 why is
the Nighthawk X10 R9000 | AD7200 Smart WiFi Router firmware still using linux kernel v 3.10.20? V3,10.20 was released in 2013. Why would a premium product released in 2017 have firmware based on a linux kernel from 2013? version 3.10 was was no longer supported 2 years before the r9000 was released