Theories on why IMDb message boards were closed (and wiped)

This. This is telling. I think it was that they got tired of hearing from users, sponsors, and maybe even actors, directors, and producers (more likely people who represent them) about all the negativity there, so they simply disabled the feature.

That seems most likely. Probably suffered one too many phone rants courtesy of Les Grossman or some diva's agent about her real age. Stupid stuff.