Well, perhaps this question would be better phrased: "Why is Exchange so cheap?"
Feature
Zimbra/Exchange
Mashups & Zimlets
Yes/No
Docs with SpreadSheets
Yes/No
Open Operating Systems
Yes/No
Instant Messaging
Yes (beta) / No
Standards complaint
Yes / "What's a standard?"
Advanced Search
Yes/No
Mobile HTML Client
Yes/No
Apple iSync Connect
Yes/No (none that I could find)
Convert docs to HTML
Yes/No
Now, Why is exchange cheap? Well, Just ask Microsoft (see here:
Microsoft's curious infatuation with Zimbra, redux | The Open Road - The Business and Politics of Open Source by Matt Asay - CNET Blogs
"Microsoft lists several Zimbra strengths that sound much more compelling than its defense of Exchange 2007:
- The open-source-software components [used by Zimbra] are proven solutions by themselves, many with large existing user bases (MySQL, OpenLDAP, Apache, PHP, SpamAssassin).
- Its' class of service architecture allows for simple tiered access to services.
- Searching via their Web interface is fast and robust (AJAX query builder).
- The Verity HTML converter allows for fast rendering of 200 documents to HTML.
- Zimlets, can provide a more interactive user experience compared to the Microsoft equivalent of SmartTags.
- The Documents solution provides integrated wiki-document sharing with powerful embedded document features.
- The AJAX interface is pleasing and has a "wow" [The wow is now?] factor due to speed and Zimlets. Additionally, its' [sic] interface is identical across Internet Explorer/Firefox/Opera.
We don't provide a product that sacrifices the user experience. If you're interested in the cheapest, then there are many options. If you want the best, then it's Zimbra.
BTW: They the spreadsheet is a bunch of FUD. They assume you're not upgrading for 4 years, which is fine for windows users. We all know how long it takes to get a New release. However, each time, you pay a license cost. With Zimbra, upgrades are free.