Guns will be allowed immediately outside the convention center in the outer perimeter surrounding the Secret Service-controlled area due to Wisconsin state law.
It doesn't give a reason, but lists their ban alongside "paint-ball guns", so I'd guess they're simply a nuisance - in that you could biff them on stage without much risk of rebuke.
Hmmm... Y'all remember when there was a mass shooting at an RNC softball game? Iirc nothing came of it but I certainly feel like these gatherings are kind of... prime targets, no?