There are several reasons.
1. We had a support case where HP Agents were causing purple screens on our hosts. VMware support asked us to uninstall.
2. Meeting with HP Engineers at HP's Lab center in Atlanta we were told to NOT install HP Agents on an ESX Host
3. Our VMware SE specifically said do not install HP Agents on ESX Host if it can be avoided.
4. Former HP Top Engineer now with CISCO in a meeting with CISCO said, do not install HP Agents in COS.