Several DOE sites won't even consider Singularity, including NERSC, who created Shifter (aka Docker + patching - networking) around the same time as development at Singularity started at LBNL (NERSC's parent lab). LANL created CharlieCloud (https://hpc.github.io/charliecloud/) because they needed a hard focus on security first.

There is quite a bit of distrust of Singularity places that value security. I'm hoping podman rootless can eventually come through and render all the bespoke implementations moot, but it will probably be a few years.