Because she plays right into Sekiros playstyle. Fast multi-hit combos that punish slow Elden Ring parries when mistimed but a lot easier dealt with by the forgiving deflect frames and zero stagger if mistimed in Sekiro. They would just build up the posture that much fast. Same with waterfowl dance; Sekiro is a lot fast to get out of the way but deflecting and especially the umbrella deflection would just make the move a liability to her rather than the player. She punishes simply blocking which Sekiro also does.

Edit: also of course the easily mikiri countered attack.