Board Thread:General Discussion/@comment-31371445-20170222233857/@comment-31536324-20180621185528

Monsters have SOULS that represent Magic, Love, and Compassion, and given how they've used attacks colored after the traits, it's not that farfetched to assume that not only is each one the magic of each trait, but that Monsters can use all types of Magic (as Undyne proved, Monsters CAN use Determination, it's just that they can't handle it). They have variety, but lack power due to being naturally weaker than humans.

Humans however not only have just seven traits and seven magic types/modes to work off of, but they can only use one naturally per individual - unless the Mode is forcibly changed like with the Undyne and Muffet fights. What the individual lacks in variety, they make up for in power.

It does make sense when you think about it.