I'm guessing this is because they've been going after his items.
AP Malphite, for example, seems to be pretty balanced. His most purchased AP mythic in Luden's is ~51% win rate. Considering the last three patches chain-nerfed tank items, it makes sense that they tried to hold off on nerfing him specifically while hitting items that needed it.
That said, Malphite is still broken and should be nerfed.
Ye you prob right. We didn't swing at many of the tanks (except Amumu, that guy was wack) because we planned to iterate a few times on tanks. That said, we will be wack-a-mole-ing properly now that pre-season is coming to a close and items are getting more stable. The fact that only a few champions are outliers like this means we are getting close to a (relatively) balanced game!