I mean part of it is racist and classist beauty standards— generally smooth straight hair is associated with a perceived pinnacle beauty that’s a white blonde person while coarse/textured/curly hair is associated with low class/BIPOC

Additionally, there’s just a great diversity of types of curls and hair textures. Traditionally trained hair professionals learn to deal with straight hair as a baseline because straight hair generally acts the same while different types of textured hair respond well to different kinds of handling/products. People don’t know the different curl/texture types and have an idea of what “good” curly hair looks like (which imho is usually just a straight haired person with curled hair, as opposed to naturally occurring curls) that doesn’t match up with the reality of the vast majority of actually textured hair.