It wasn't "super random", actually. The additions of music are related to added content, new games that came out after smash 4, and, of course, fan demand.
For example, we got a Metal Gear 2 remix to go with Gray Fox's return, Central Highway, X vs Zero, and Theme of Zero (From MMZ on GBA, you can hear the fuzz) to go with Zero, etc. I really, really do not see why you would not add just one more Golden Sun rip when you have 7 spirit battles, or, at very least, not explicitly limit yourself to Golden Sun music. You know, like every other spirit set does.... except for Xenoblade 2!?!? Huh...
...and I don't understand why we still go off of this logic of "it has to be relevant!". By all accounts, if this was true, we should not have Banjo at all. This is the definition of a character that Nintendo does not have any interest in including except for fan demand, and yet they did.