Basically, the current worlds of each creature were failing. Either important resources were running out, some kind of natural disaster had occured/was approaching, or the sun was about to explode. Whatever happens, the worlds were dying.

Suddenly, a benevolent god decided, as all loving and merciful gods do, to save the races by transporting them to the EoFFworld. The planet was especially created to harbour life, and had regions suitable for all. Using his mystical powers, this god (who I'll name Alathan, but could be given different names by different races) sealed the five worlds in time, and transported their inhabitants to the EoFFworld.

That would be a pretty standard viewpoint. More unusual or extreme beliefs might be more interesting.