FreeRTOS isn’t an OS. It’s a nice scheduler. Which is what the vast majority of people want and need when they think embedded.
Zephyr, is an OS. And imo will be eclipsed by embedded variants of Linux quickly as chips are coming up in performance. There is just no path for co-existence I see.
Zephyr, NutXX, and others aren't GPL, and that is a big deal for those folks.
Zephyr and Linux are not competing, generally. There are literally billion devices shipped annually in Zephyr niche - microcontrollers with under 1 MB RAM. This will continue the next 10 years - as for significant of the market cost reductions will go into power and radio improvements - not spending money on RAM and FLASH to run Linux.