Maybe because Kyaa's a different person, and she has her own preferences and desires about how to look or how to wear her hair?

Also, what an odd idea to hold Nym accountable to explain Kyaa's choices.

Kyaa's hair is long and beautiful, and she is a sexy woman with wonderful scenes of extreme hardcore sex. Nym's hair is also beautiful, astonishingly long, and she performs amazing hairjobs. By all means, enjoy both of their work. It's there for your pleasure.