Board Thread:General Discussion/@comment-29788596-20160906011125/@comment-32182236-20200129173725

Because nothing can cross the Barrier, and we still get to receive the call.

Also because Frisk couldn't have taken Asgore's SOUL, and we all know it takes a human+boss SOUL in order to cross the Barrier.

But if Frisk did cross the Barrier, then that's Option 1-Meaning Neutral Runs would be fine for Chara's plan, so it makes sense why they only said "another path" while not explicitly telling you to not kill a single monster.

If Flowey hijinks is what propelled Frisk beyond the Barrier, these hijinks don't happen in Genocide, so Chara still has a reason to call it a dead run and destroy the world if Frisk goes Genocide.

And Flowey hijinks is really the only possible way Frisk could have crossed the Barrier-They didn't have a Boss Monster SOUL to use to cross the Barrier normally.