"They also DO NOT nominate show that they DON'T LIKE or that have annoyed them."
That's a good point and certainly helps explain YF and TLM being omitted, but it also may explain why ACA did not get the fourth slot. It wouldn't surprise me (though I admit I'm totally speculating) if the nominators were split on ACA -- that is, perhaps some really liked it and wanted to nominate it, but other nominators not only didn't like it, they really HATED it. Even if nobody really liked CB all that much, it could be that it was just less objectionable than ACA for some nominators. In other words, maybe CB was just a compromise -- it was the least objectionable to the most nominators.
The reason I think this could be possible is just thinking about how the shows were treated by the reviewers -- ACA had that glowing Barnes review and a couple other nice reviews, but it also got panned badly by a bunch of others. I don't recall the reception being as divided for CB -- while I can't recall any rave reviews, and while I'd probably say most of the notices were mixed or negative, on the whole, they didn't seem as harsh in their criticism as the worst of the ACA reviews.
"This is wanting something, this is reaching for it,
This is wishing that a moment would arrive.
This is taking chances, this is almost touching, what the beauty is." --The Light in the Piazza