Supply and demand. In general, people like to play DPS more than Healers or Tanks because things in society like TV, Movies, Literature, and other games ingrain in our minds that it's cooler/stylish to be offensive rather than supportive or defensive. Main characters are rarely a healer or a defender, they're usually running into the thick of battle and killing dragons or severing heads. Being a DPS feels like you're affecting the enemy's death directly. So, psychologically, players of video games almost always want to be a DPS character. Devil May Cry's main character has 2 guns and a big sword. How lame would it be if you just played as a Healer and you run around healing Dante instead of BEING Dante? Exactly. Blame psychology and social reinforcement for your long DPS queues. Or, play outside of the mold by playing a healer or tank and enjoy fast queues.