There are a few reasons why backpacks are not allowed in stadiums. The first reason is that they can be used to smuggle in contraband. Contraband can include things like drugs, alcohol, and weapons. Backpacks provide a way to sneak these items into stadiums undetected.
The second reason is that backpacks can be used as projectiles. People have been known to use backpacks to throw objects at players or officials. This can be extremely dangerous and has resulted in serious injuries.
The third reason is that backpacks take up a lot of space. Stadiums are already crowded places and adding more bags to the mix only makes it worse. This can create fire hazards and make it difficult for people to move around freely.
Ultimately, the decision whether or not to allow backpacks into stadiums is up to each individual stadium. However, most stadiums have banned them in order to ensure the safety of everyone involved.