The total cluster load is high.
When the total cluster load is high, DRS does not power on the host.
The following are possible reasons why DRS does not power on the host:
VM/VM DRS rules or VM/Host DRS rules prevent the virtual machine from being moved to this host.
Virtual machines are pinned to their current hosts, hence DRS cannot move these virtual machines to hosts in standby mode to balance the load.
DRS or DPM is in manual mode and the recommendations were not applied.
No virtual machines on highly utilized hosts will be moved to that host.
DPM is disabled on the host because of a user setting or host previously failing to successfully exit standby.
Address that issue that prevents DRS from powering on the host.