The prize of Christianity

[Neo-Scholastic]

...I'm having trouble envisioning a reason why one wouldn't be able to just reduce the question from multiple gods to one god.

In Greek polytheism the various gods and goddesses acted capriciously and often at odds with each other. As such obedience to Zeus, for example, would defy Hera. Each god having its own distinct nature gives rise to the dilemma. Since a monotheistic God has only one nature, the good is based on God’s inherently good nature. God does not ignore his own nature when he declares something good. Nor does He need to refer to any standard outside Himself. He serves as His own standard. Thus there is no dilemma.