Although I'm not 100% sure why.

It could just be that as Zimbra is based on the legacy appliances (rather than the latest release) it was too much work. I did a lot of the early dev work on creating OVZ templates and there were significant changes between legacy TKL and v11.x. My notes on legacy OVZ template creation should still be in the dev wiki though if you want to have a crack at creating one yourself.