Why tank supports?
Its simple, really.
As a support, you need to get as many spells off as possible for maximum efficiency.
You can do two things to assist yourself in doing this.
A: Build cooldown reduction to increase the rate at which you output spells B: Build tank stats to increase how long you can fight without dying.
A combination of these will work wonders, which is why most popular supports are high utility tanks (or general high utility champions built tanky).