There are 2 BIG reasons:
1. Bloom Diva the Melodious Choir, cannot be destroyed by battle and card effects; can destroy opponent's special summoned monster. Imagine having Cocytus and Yubel - The Ultimate Nightmare's effect in 1 monster.
2. Score the Melodious Diva. Basically Crimson Fox version 2.0 but without limit 2.
Bloom diva can be targeted, hence can be banished where as cocytus cannot be targeted nor destroyed even if not targeted by card effects, though bloom diva has battle damage suppressing effect.
also bloom diva dos not have yubel's floating effect after leaving the field.
also yubel the ultimate night mare will destroy any monster it battles regardless of summoning methods.
Because the Skill. They can summon the handtrap to swap it for a big monster with no cost who recycle it and keep making their field. Many other decks would 🔥 for this type of skill.
With protection from destruction effects and being destroyed by battle, Bloom can stall and dish out damage when attacking. That thicc fusion can clean out GYs and Score can drop an opponent’s monster’s A/D stats to 0. To make matters worse, Melod decks can run TTH too. The deck is broken.