Hi There. A number of factors.
1) FreeRTOS is ineffective, in many cases you have to code around the kernel to get sufficiently good response times.
2) FreeRTOS does not offer any means to schedulability analysis, as the internal overhead is very hard to predict and data dependent (and lacks proper characterisation).
3) The threading model as such is not particularly suited to concurrent programming (RTFM is event driven, reflecting the reactive nature of the hardware as well as the application).
4) Thread based programming is hard to get right, see e.g., https://www2.eecs.berkeley.edu/Pubs/TechRpts/2006/EECS-2006-...).
5) Regarding FreeRTOS, the current license disallows to make comparisons with FreeRTOS without their permission, how about that for starters!!! /Per