Pretty much all projects/libraries (even newer ones) seem to build upon the IoT SDK instead of the FreeRTOS SDK. Why that? The FreeRTOS SDK is undoubted far superior as it directly exposes the underlying open source libraries. The IoT framework provides only very thin wrappers around them which don't make programming easier but harder as the are very poorly documented and don't even provide the full feature set of the underlying libraries. Also Espressif seems to close it down more and more with every release (as the did for example with lwIP in v0.95).
But maybe I'm missing some point. I would be really interested in hearing what's the reason so few people use the FreeRTOS SDK.