Board Thread:General Discussion/@comment-27907368-20160506131212/@comment-26887879-20160513201758

GetYourFix's idea makes sense... Maybe when facing Asgore, their final challenge, the previous six humans were driven buy their key trait to defeat him and like when Frisk battles Asriel, their Souls would refuse to shatter and reatach. But, that was in fact bad for them because, unlike the case with Frisk, there weren't enough Souls to destroy the barrier...

And what back ups this theory is writing at the Waterfall about how Human Soul's persistence after their death is at the same time their weakness... Basically, Frisk was just lucky they were the seventh Soul.