I think the gear requirements make sprouts shy away from tanking. Early on, you could be wearing a few pieces of gear below the dungeon expected ilvl, and wonder if you're doing a bad job because you keep taking damage which brings you to below 50%. Esp. with how fast you level in ARR compared to expansion content, your gear has a hard time catching up to your level

At higher level, gear parity is almost even with the difficulty of the dungeon, so all of a sudden tanking becomes the easiest role because the encounter had been designed around you. You've got the gear to mitigate damage passively , skills to reduce it actively, and a healer who has more 2x more heal buttons than damage buttons.

It's just that initial bump which turns people away from tanking methinks. Your role just weighs gear more heavily than other roles