Maybe it's like the conference speakers. Now that everyone's getting flack if their conference doesn't have female speakers, but there's only a handful of successful female leaders in heavy rotation in the media, they can't go to everything so they inevitably turn stuff down.
Edit: I'm asking because @dnautics gave us an anecdote that doesn't add anything useful. Knowing why would be much more interesting, although still an anecdote.
I discovered that this is not the case. In general, Board meetings don't work like hollywood says they do. Usually my board meetings are pretty uncontentious.