Board Thread:General Discussion/@comment-30388913-20170828193135/@comment-32182236-20170828233425

Monsters cannot absorb other monster SOULS, just as a human cannot absorb a human SOUL. Humans cannot absorb monster SOULs because they turn to dust at death, and it would take an incredible power to take the SOUL of a living monster. They CAN absorb Boss monster SOULs, as they persist after a few seconds.

Flowey manages to absorb the six human SOULs and all the monster SOULs because he is neither human nor monster, and can thus absorb both of them.