Why?

Trust issues, mainly.

I don't want to tank for randoms. There's too much pressure in the expectation to do well or near-perfect, even if you're not accustomed to tanking. I am working on a tank, so I can tank for friend, but they know I'm not the best at it, that I'm learning how to improve, but at the same time I'm not going to stress myself out over it. I'll learn as I go, I'll improve by doing and making mistakes, failing and recovering.

But that's not good enough for people on the Duty Finder, and I'm not wanting to ruin my time dealing with the fallout from it.

So that's why I tend to avoid the DF as a tank.