The trouble is, EPEL packages, including snapd, are built against RHEL8. However, CentOS is lagging with the updates at this point. Even though the relevant package from RHEL was clearly rebuilt https://koji.mbox.centos.org/koji/buildinfo?buildID=5028 it wasn’t pushed to the distro. It’s probably best if you inquire in CentOS related IRC channels or mailing lists as to when the update push might happen.