I suppose the main reason for using Active Sync is that most smartphones today come with that functionality included and it requires little or no effort from the Zimbra developers to put it in the mobile devices... just the server side. And it helps non-techie end-users get connected to their Zimbra server without doing much.
...it may also help to move companies' servers away from MSEx without changing the user experience.
But, in any case, it shouldn't be too difficult to come up with a "native" app for the Android platform. For example,
LVContacts Sync for Zimbra - 101 Best Android Apps, or
Platforms; there's bound to be others out there.
((And, just to clarify. Zimbra Desktop is based on Mozilla Prism which,
AFAIK, uses Gecko as the rendering engine, and the main language is Javascript, the J in AJAX.))