It stems from the fact of how easy they are. You honestly do not need a healer or a tank in low level instances at all.
Even the current 90 heroics are so easy I have soloed a couple of them on my elemental shaman (was ~525 ilvl at the time so not super amazing gear). I could probably solo all of them if I actually tried.
It is hard to be patient and wait for a slow moving tank when you can easily solo all the "heroic" 5 man content in the game.