Those not following levelling guides might never even go there, and those who were following guides hit the zone for those few quests and then never returned.
Because there were a handful of forgettable quests and nothing else of interest. Those of us who remember the actual vanilla Dustwallow Marsh. In patch 2.1 (I think), which is definitely not vanilla according to most purists, Blizzard added a bunch of new content to the old world including several new quests around a crashed zeppelin in the previously unfinished and very boring Dustwallow Marsh.