Board Thread:Questions and Answers/@comment-5956954-20151218115701/@comment-26506863-20151221063155

Ok, i think i got it:

So, if all the monsters could just gather up in one spot and destroy the barrier, then they would've, but they can't.

The reason is that all the souls have to be in one individual. In order to just pass the barrier, you need the soul power equivalent to one human soul and one boss monster soul combined. Alphys explains this pretty neatly just before the Asgore fight. So, it would take one monster a hell of a lot of killing to get a soul powerful enough to break the barrier. If the barrier could be broken by, say, two semi-powerful monsters, why wouldn't Asgore just go along with a human to break the barrier with them?

Flowey breaks it at the end of true pacifist since he's got all the souls in one place. The souls aren't used up or anything, but they still break the barrier.