rkt (and many other container solutions) was introduced after docker was released and became popular... they even mentioned docker's shortcomings as a motivation for the project creation [0]. It had all the same problems as other replacement software: there were plenty of bugs and missing features (the announcement mentions "prototype quality release"), documentation was limited, and there are no community to help you. None of those would be fatal if it was significantly better than docker, but it was not -- it had less functionality and needed more scaffolding. So almost no one made the switch. It is closed now [1]

And why "rkt"? There were much better alternative container runtimes. For example Sylabs Singularity [2] -- container-as-a-file, instant mounting, etc... I wish more people knew about it.

[0] https://web.archive.org/web/20141201181834/https://coreos.co...

[1] https://github.com/rkt/rkt#warning-end-of-project-warning

[2] https://github.com/sylabs/singularity#singularityce