It must have just recently been discontinued. The MT-16 is not currently listed on Pirelli's website. There may be some places that might still have some MT-16's still in stock and may have it still listed on their website, but the following is the e-mail I received from Rocky Mountain after trying to order.
We are very sorry but the 110/100x18 Pirelli MT16 Intermediate Terrain has been discontinued and we will no longer be able to get the item. Since there are no other items on the order it will be canceled. We apologize for any inconvenience this may cause you.
Rocky Mountain ATV/MC