Why don't more people use Nasus support?

Why don't more people go Nasus as a tanky support? It works really well now. The Face of the Mountain line of items is perfect for building Q -- you get one minion every wave (including siege), plus you farm extra while your ADC is backing to town buying items. Also, after laning phase, you transition into tanky nasus who can free-farm since you don't have a specific laning opponent. Additionally, wither and spirit fire guarantee that your ADC wins all trades. Everyone knows how fun it is to be the jungler on a team with top-lane Nasus -- it is free ganks. Now your ADC is like a jungler -- trade for days!
You basically build standard tank Nasus, but you throw in a sightstone and face of the mountain before getting your sheen item. You will be tanky, deal tons of damage, and you will have shut down the enemy carry from the very start of the game.