To say that no one had heard of the Doom Patrol is pretty misleading, I suppose, but the Doom Patrol has never really been that popular. Sure, people have heard of them, but if you live in the DC universe, your first choice for world saving will always be the Justice League. And if you're reading Doom Patrol comics in this post-Morrison age, you're definitely not coming to the Doom Patrol for straight up superheroics -- you're coming to the Doom Patrol for something offbeat, something strange. The superhero stuff is just the chaser.
It's fitting then, that the creators of the Doom Patrol didn't really come from the world of superhero comics writing. Arnold Drake wrote It Rhymes with Lust, and Bruno Permiani was an Italian romance comics artist. The Doom Patrol is a superhero team whose creative roots come from pulp/noir and romance -- stories that deal with regular people, not necessarily heroes. Cliff, Rita, and Larry are regular people (even though they may not look like it-- Cliff is a brain in a robot body, Larry is wrapped in bandages, and, conversely, Rita is a beautiful movie star) but they are regular people who are also heroes. Not so much because they chose to be (at least not at first), but more because of the circumstances of their afflictions.
Arnold Drake's Doom Patrol may look like an expert team of superheroes, but the uniforms still feel ill-fitting on them. They don't call each other by their codenames (in fact, when the shape-shifting Madame Rouge infiltrates the team, Robot Man finds her out when she calls them by their codenames), they agonize over their ruined civilian lives, and ultimately I don't think they're fighting to save the world-- rather, they fight to save themselves. The Doom Patrol is a team because they give each other a reason to keep fighting. Without each other, it's easy to imagine the members of the Doom Patrol sinking deeper and deeper into their own depressive states. Being a part of this superhero team gives Cliff, Larry , and Rita something else to think about, some other external conflicts to keep them from their own internal conflicts. In the end, the Doom Patrol are not a team of superheroes, but a group of friends-- people who need each other, people who realize that the traumas of life, much like evil brains, bloodthirsty gorillas, and time controlling despots, can't be faced alone.