Originally I thought that there was some kind of power or strength quota - that only a certain number of individuals could be all right on the barge at the same time. However, I don't think this would hold up to examination, as barge coma patients do wake up after others are welcomed on. The other thought is that each person has some sort of spell or power allowing them to exist extra-dimensionally, and when that spell or power fails, the person enters a barge coma as protective measure. But some beings, myself included, are able to exist extra-dimensionally on their own, so if the spell failed, there wouldn't be a barge coma, but that wasn't the case, at least in my experience. Of course, the power of the spell could work regardless of one's species, which is a consideration.