Yes that is because your HA cluster is set to ensure that it has enough resources that in the event of an ESXi host failure it can power back on your VMs on another host. This is the HA admission control feature of vSphere. You can either power some other VMs off, or perhaps change the admission control policy - there are various options, from having an extra host to preserving a certain percentage of cluster resources for a failover.
Where things get complicated is that HA works according to a 'slot size', which is basically the size of your largest VM, it may be that installing VE has increased this to 2 vCPUs and 4GB RAM, which is what has effectively filled up your cluster, even though there may be plenty of resources available.
Take a look here for further info http://www.yellow-bricks.com/vmware-high-availability-deepdiv/
Simon
Nimbostratus
