Due to the increase in feature complexity and script optimisation, many object attributes must remain untouched to avoid breaking overall functionality. This paired with a significant decrease in things you can change with modify permissions meant it was no longer feasible to offer modify permissions.
Example: Modify permissions no longer allows you to change the textures or materials, as these are made with a custom layout made specifically for the hair. You're also unable to remove, hide, or move, any individual hair strands as hairstyles are now one object with one object face. Note that this is not a complete list of things you can no longer change in-game on Magika hairstyles.