Because it's a level 25 FP and most people have no clue how to play properly at that point, especially when the dogs should be kept away from the boss who the tank should be beating on (the boss has an aura that increases the dog's damage so they need to be separated). People tend to be idiots, especially when it's lol-leveling content. If a strategy isn't readily and *obviously* apparent, most people couldn't be bothered to figure it out or even to follow it if it's spelled out to them.
Thanks for the info on the fight. Sadly it is one of those fights where the strategy isn't "obviously" apparent. Add to that, people still learning to tank/dps/heal, don't have their full compliment of tanking, damage and healing abilities, and there is no HM version of the FP so people will be lucky if they do it more than 3-4 times per character.