I decided not to spell out a hard and fast reason for the demons staying on Oerth. In Foot Note 2, I imply that (much like the nabbassu) Wagamawezwik's followers might somehow gain power by staying on the Prime Material plane. As for the demon lord himself, there may be more to his staying in the forest then meets the eye (Abyssal politics, personal power, the raw desire to spread chaos and famines throughout the multiverse)...
But I am leaving that explanation to those bold and brave adventurers of Oerth to discover!
