Why? Just... Why???

The SOUL shattering seems to be directly connected with SAVE and RELOAD.
After all, from what we learn from Flowey and his story, only the being with the most Determination can SAVE or RELOAD, so if the prior humans didn't have more determination than Flowey did, they wouldn't have this ability, and thus their souls wouldn't shatter.

But that would mean flowey has more determination than the player, meaning we shouldn't be able to save or reload

Flowey himself states that the player is special, and stole his abilities (until he gets the human souls and regains control)
Which might be because the player’s soul has no unique trait and is just pure Determination

Huh

Well consider that all the other souls have unique traits: Bravery, Kindness, etc.
But we don’t seem to have one
Some say our trait is Determination, but all human SOULs have that, no matter what their trait is: it’s not just a trait, it’s part of the human SOUL
So if our only trait is Determination, our Determination is therefore stronger than people with a unique trait

It must shatter immediately so that the monster doesn't have a chance to absorb it, then it has the determination to go back to its last save point despite dying.
...I dunno I just came up with it on the fly.