User blog comment:Lord Loss/Discussion of the Week: Rare Species/@comment-765166-20131201094252/@comment-6050823-20131201131806

^ Thats pretty much confirmed for Silver Rathalos and Gold Rathian but heres one I genrally accept. From their metal hides, they absorb heat faster so that's why I think they only appear in certain areas because they could overheat faster.

Anyway, the Tower's story and why it was made is still a mystery though Capcom has left some interesting hints in some quests in the Towers. Some of the monsters that appear in the Towers have their reasons. Another thing to note is that the Great Forest is close to the Towers so, its possible that afew of them came from the area like Mi Ru and the Lucent Nargacuga. You can even see the Great Forest(Or another area)down below while fighting the Silver Rathalos/Gold Rathian in MH3U.