Why can't middle schoolers carry backpacks?

I'm curious, why is it that middle schoolers are restricted from carrying backpacks? Is it a safety concern? Or perhaps a way to maintain a certain level of uniformity within the school environment? It seems like a practical item for students to have, so I'm wondering what the rationale behind this policy might be. Is there a specific weight limit or type of backpack that's not allowed, or is the ban applied across the board? It would be helpful to understand the reasons behind this decision in order to better support the needs of students.