Vmware 5.0 and aboce support the FDM model for the fault detection. And also selects 2 shared datastore as per your configuration to get the heartbeat. in your case You removed the network for one of your host then the host lost the network heartbeat, but meanwhile the datastore was accessible to the Host with datastore heartbeat. Thats the reason your HA was not strated the failover in case your lost the network and datastore both connectivity (Like power outage, Host or power supply Failure etc. ) then your HA will activate coz both the heartbeat are not receiving.
Hope you got it why your host not initiated HA in your environment.