My issue with tanks is that they're often awkwardly more difficult to play as in singleplayer when you don't have a healer with you.
Healers can usually heal themselves, keeping them going.
DPS in many games have a health-stealing ability, or are more capable of avoiding damage long enough to heal.
Tanks though are less agile, often can't heal themselves well, and don't do quite as much damage as DPS, so take longer to defeat enemies.
Also, they're usually your more vanilla character in most games.
Typically your heavy armour, and sword and shield sort of guy.
Compared to your sneaky, tricky DPS guy (who may also be ranged). Or your magical or elemental healer type.
Typically I go DPS, but I end up sort of tanky by means of poor skill allocation.