"The distinction between foreign and local forces simply doesn't exist in Monster Society. The underlying reason for some monsters' resistance to Zhou Tian ultimately boils down to the threat they feel from his presence."
Although Zhou Tian didn't immediately start expanding upon reaching the Mainland, there was no doubt that his strength was already comparable to most King Level bosses. With the power he demonstrated, if Zhou Tian had wished, he could have established a foothold on the Mainland. To do so would have required seizing territory, which could only be taken from other Monster Forces given the static size of the Mainland. Under such circumstances, it was fortunate that Zhou Tian's current power was respectable, otherwise those Monster Forces probably would have driven him out long ago.
As per the common nature of monsters, where there is benefit, the worst of enemies can become friends, and friends can just as easily become foes.
