Psychic/Steel make more sense than you think, it simply requires a high IQ to understand the choice. You see, steel is commonly referred to as Iron in the games as they are similar enough to confuse, but what is the importance of Iron? Well iron is the last stage of fusion in supermassive stars, fusion is the process by which gravity pushes atoms together to create denser atoms and energy, which allows the star to keep it's form, but iron is the final stage of fusion. But it doesn't stop there! Gravity in Pokemon is represented by the psychic type as seen by the move. Meaning that Solgaleo represents a collapsing supermassive star which has an iron core.
Gamefreak's intelligence never ceases to amaze me, I wouldn't expect you to understand.
