Conditional acceptance then rejection – worthy of appeal?
I believe you have misunderstood what the first "acceptance" meant (which could have been worded better to prevent this).

A journal generally has many editors and an editorial board. 

The typical journal review structure has your paper go to a single editor, who decides if the paper passes first muster and assigns reviewers if so.  Once the review process is complete, that editor then decides if the paper is to be rejected, revised, or advanced to the full editorial board. Your "conditional acceptance" seems to be the first editor advancing the paper to the full editorial board.  The board then made the final decision to reject the paper.  Note that some journals (in some fields) can have a high rejection rate from the editorial board. So while that initial "acceptance" can feel like a victory you're not actually out of the woods yet.

I expect your chances for a successful appeal to be small.  The decision seems only slightly based on your paper and mostly on the technical details of how the journal selects publications.  They're unlikely to bend those to your benefit.