I personally think tanks aren't really in short supply. My FC, static and LS's are full of them. Mains and alts alike. I have both tanks myself. They are always needed and they know this. And they're always employed by FC or other group members, which leaves only a few group-less or solo ones that actually queue for DF. Because other than the roulette bonus, there is very little reason someone would use the DF as a tank. And why would they want to help 3-7 other randoms, when they can help their friends?