"They" didnt fail, only Bruce Willis.
As stated in the "future", J.C. was never sent back to prevent the outbreak - that was a delusion that he came up with on his own brought on by the mental stresses of the time travel shock. He thought that he could change the past, when really it was an impossibility. Proof of his delusion is his confusion at Pitt's aniimal activist rights group "Army of the 12 Monkeys", and being distracted enough to think they had something to do with the virus.
Unlike the previous posters, the term however is not "a loop" , its more of a paradox, that J.C. would actually witness his own death as an adult when he was a child in the past. Its a paradox by the definition that has he suceeded, and not gotten killed, then he wouldnt have witnessed his death a a kid, and would never have suffered the dreams of it and eventually become basically criminally insane in the future (and therefore would not have been imprisoned to be sent back to suceed OR fail).
The scientist panel of the future only needed to know more about the origin of the virus in order to help begin finding a cure, by understanding its epicenter of outbreak.
Then again, I feel the ending is left open for subjectivity, so in that sense, you should believe what you want to about what happened.
If you really want to make your head swim, there is always the theory that the "future" end of things never happened... that was the paranoid delusion. Go back and watch it with that in mind, and you'll see some interesting clues that point that direction!