You don't get the good rewards if you don't meet the point requirements is the short answer.
Long one https://flowerknight.fandom.com/wiki/Flower_Memory
"Each stage in a dungeon has a Recommended Memory Score / 推奨メモリースコア. If you meet or exceed that recommendation with the combined memory score of all characters in your set, then clearing the stage gives you Memory Crystals / メモリークリスタル alongside the usual memory stones. Like the memory stones, the memory crystals are used to buy things from Wares."