Why do so many humans struggle with the concept of compassion?

[Sacred Fool]

People who are not negatively polarized, who sometimes believe themselves to be religious or constructive members of society, seem to still struggle with the concept of compassion for other-selves.

It seems to me that your answer is sitting right there within your question, if you view it in the context of hierarchical levels of consciousness.  So, a person who has learned well to function at the level "self-as-social-being," but has not learned to function as "self-which-embraces-universal-love" will be in exactly the place you describe: socially functional, but not yet attuned to universal compassion, no?  Yes?

Compassion for dedicated STS beings is folly from a 3d perspective, so as long as I have to operate in a 3d environment, STS can do what they do best and "love" themselves

You may be headed down the road of unintended consequences.  An STS being who can provoke a person into taking such an attitude has created a follower, in some sense.