They have the most health, the most defensive CD's, just as much healing as anyone else (Even more healing if they're a warrior) they can become completely immune (PLD) or heal for the amount you smack them for instead of taking damage and dying (DRK.)

So what makes you think it's a good idea to target the nearly unkillable thing first?

No. Kill the squishies first and turn a 5v5 into a 5v2. THEN kill the tanks, when you can just burst them all down.

Eugh I am so sick of not being able to communicate in CC. Give us teams already!!!