If it were single-user, that'd be one thing, but given that the whole shtick with Trello is that it's collaborative, you find really fast that there are almost zero real-world situations where you can actually do something sane when actions diverge. You'd end up building a DVCS into Trello just for cards, and then having a hilarious conflict-resolution process. That goes pretty strongly against Trello's KOSS ethic.
(I don't speak for the Trello team, but I remember trying to reason out whether we could do something similar on FogBugz, and rapidly concluding that the real answer was "absolutely not.")
Are you telling me that enterprise grade version control, powered by machine learning about my organization's workflow is out of scope for a small, simple, free service? ;-)
You're absolutely correct though - yes - this quickly becomes a way bigger issue than I originally gave it credit for in my HN-esque knee-jerk comment. I'd still love to be able to leave comments on a card though.
I've always liked the idea of allowing offline commenting (and card adding, for that matter), but I would be worried that 1) users would be confused why some things can be written offline, but not others, and 2) since so much of Trello is about the synchronization, it might be weird to have that kind of de-sync while the user is offline.
I would assume that it's because conflict resolution is a difficult problem to solve. I think it would take a lot of work to craft an interface that allowed managing conflicts easy, especially for the non-technical members of the team.