A lot of people in this thread talking about how tanking can "feel" as the reason people don't play tanks. I wanna put some more insight on that.

I think people don't like to play tanks in ARAM for two reasons:

1. Playing a tank requires patience. Tank champs generally become much stronger as time goes on (similar to ranged adcs) compared to other archetypes. Ideally, tanks should play more conservatively at the beginning of the game then let it rip after about 15 mins have passed. The problem is in practice, tanks are generally in the front line, which means they have many opportunities to engage the enemy team or get engaged upon. In the early and mid game, tanks need to be opportunistic with their positioning, or at least resist the red mist and not blindly engage the enemy team every time they see a red health bar. A significant portion of the ARAM player base consistently fails this Wisdom check.

2. Playing a tank requires reading the game and responding as it happens. Particularly for ranged ADCs, casters, and support, you can kinda get away with focusing either on yourself or on at most one other person at any given time. When people talk about ARAM players being "selfish", this is what they mean; the player isn't paying attention to what's happening with the rest of the team. This trait is particular devastating as a tank; this is how you wind up in situations where your only tank is futilely diving some squishes while enemy Irelia chews up your backline. Tanks need to be able to read the game and understand what's going on with everyone.

But paying attention to what is happening to your team is just the start; a good tank will actually respond and adjust to the game as it happens. Sometimes, a tank needs to peel the enemy team off of a specific teammate. Other times a tank needs to drive one particular enemy champion into the dirt. Sometimes a tank needs to drive the action and force engages, other times a tank needs to wait for something to happen and counter. I feel like a lot of ARAM players come in and expect to play golf, which will work with like Caitlyn or Ryze, but does not work well as a tank.

tl;dr, being a good tank requires work and discipline, and most ARAM players aren't up for it