Honestly its because tanks and healers typically are social creatures. I know I love being a healer because i love the social aspects and responsibilities that come with it. And as you get known on your server as being a reliable healer or tank the chances of you even needing to queue for anything diminish. For example in daoc, eq2 and wow when i logged in i had messages within a few moments asking if i had enough time for a dungeon run or what not from people i have healed in the past or my guilds. Healers and tanks if they are good and social rarely if ever have to queue just because they have a core group that always supplies them with groups as it is.