Because it's simply not a suitable place for an animal that can be unpredictable under stress. You dog freaks out because of the stress of fireworks, large crowds, strangers, new place, etc. and suddenly flips the script. Not a bad dog, but an idiot owner. Your dog bites someone. Now what?
Unless you're willing to accept having to pay 700 bucks to kill your dog afterwards and pay whatever costs come from any treatment for the victim, and be barred from pet ownership, don't bring your animal to public events for humans.
I hate this argument "my dog is nice" TO YOU.