1. I guess they didn't want to animate bun as superhero
2. Because superhero look is in parallel to holder's original look and are used as disguise, which means that even if LB will get suddenly bald, her hair would return unless we will get explanation how supersuits changes.
I guess it's just Ladybug's signature look. If that is the case, maybe it changes only when she becomes an adult or changes as a person (Plagg saying "This is what you really wanted deep down"). In the future, however, some people even said she would loose her pigtails bc short hair.
I mean if they kept the bun, they would probs have to make another model. (And that's painful) There are also other examples but I aint gonna get into that.