For a long time you couldn’t run Docker in rootless mode. So Singularity/Apptainer was developed so that the benefit of containers could be used with shared HPC clusters. Podman also didn’t exist at the time.

I don’t know why we should have waited for docker to get around to supporting a more secured use-case.

With Docker, security has always seemed like an afterthought, which seemed perfectly reasonable considering their original use-case. But that’s not tenable on $$$ HPC clusters with thousands of users. So, the HPC world developed their own solution, better adapted to that environment.