A recently rebuilt ESXi host cannot be readded to the DVS as it is not listed in the Distributed Switch Add and Manage Hosts -> Add Hosts wizard.
VMware vCenter Server
When the host was not removed from the vCenter/DVS prior to the rebuild, the DVS will still see a host with the same name as attached to the switch. This blocks the vCenter from adding the host to the distributed switch.
Remove the original host from the DVS using the Add and Manage Hosts - Remove hosts wizard. Once the old host is no longer associated with the DVS add the newly rebuilt host to the DVS.