I doubt it's a trademark issue, as trademarks tend to be usage-specific. (e.g. Apple Computer vs. Apple Music) It's probably just a matter of Google feeling there are too many different things already associated with the name 'Zippy.'
(I also now have the "Rainbow" theme tune stuck in my head.)
This is used in more than just BigTable; it's used extensively inside Google, in particular for the RPC system. It's great to see it open-sourced!