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

Money is always a factor in everything, within a human society that is afflicted with terminal greed.

However, if all they wanted was cash, they would have just sequestered the message boards to IMDb Pro -or- started charging for the applicable level of membership (i.e., the erstwhile standard, free membership / message board access).

By shutting the boards down wholesale, and even erasing the forum's posts, they gave the game away about their reasoning behind the move: they (read: TPTB) did not like the content the message boards housed and promoted, and so they got rid of it.