The day Monster kind was sealed into Mt. Ebott, the mages added a little extra insurance. A curse.
Human SOULS may pass. But Human forms may not.
Young Frisk must navigate the dangers under the mountain, a little less human than before...
@Tarableart: I would say that soul does not look very healthy, I have a feeling that the dripping is not part of the equation. Something tells me that if they could stabilize it then things will be a lot better for frisk. I'm guessing of course.