Pay - if you come in at a low salary it is difficult to make fair salary. When promoted, there is often no raise, or you remain well below what an outside hire would be paid.
Technical Support has a weird culture where they hire you with the intention of you getting a job elsewhere in the company and then spend 2 years playing mind games, making people feel not good enough to move onto another job or blocking the way. Why not make support a great place to work always as its an incredibly valued part of the company.