The reason is that clearing one is the exception, not the rule. Heroics are for gearing people up to do raiding content, not to be as hard as raids themselves. I think Blizzard is going to lose a significant amount of players because heroic pugs have an extremely high fail rate. I'm a healer so if one fails, I can simply re-q and get in another one instantly. But Johny DPS has to wait 45 minutes before he can try again. Of course I'm talking about pugs and the dungeon finder here. If you get a premade going, and all of your dps is rock n' roll and nails every interrupt, doesn't stand in the fire etc.. you don't have a problem. The smoothest runs I've been in were premades with a Paladin tank and 3 dps. The Pally helps heal himself and the DPS are all kingslayers and it makes the instance really fun.
But. Big but.
Most heroics do not end up that way. Most of the time you get at least one DPS that is clueless, totally focused on numbers to care about mechanics or just bad. People are so stuck in the Wrath mindset of the healer carrying the 25k hp tank and the 2k dps. However currently the healer is only a small part of the group. We arguably have the hardest job, but we're still only 1/5 of the team. Everyone needs to be up to snuff or else you wipe. Wiping over and over in a raid is understandable. It's supposed to be like that. But not in a 5 player heroic dungeon. We should be able to clear a heroic with some difficulty, and we should be able to clear it with the group the dungeon finder has given us.
As a healer I've noticed 2 main problems with the current system.
1. There is way too much incoming damage to tanks in some fights. Some of the bosses just hit way too hard. Halls of Origination comes to mind. The first trash pull is reasonably difficult for some tanks. One time, the tank reported taking a 90k hit. And I believe him because many of these tanks are dropping like sacks of potatoes.
2. DPS has zero room for failure. This is terrible design, plain and simple. People are going to screw up. You practically need the reaction time and coordination of a fighter pilot to get some of these bosses down. Commander Springvale in SFK is one of them.
To sum it all up, yes some of the content needs to be nerfed. The sooner you do it, the sooner pugs can start having fun in heroics.