@snood,
Because the bottom line is that it is fundamentally not statistically probable and that forbidding guns is about small possibilities that are worth preventing and not probabilities that are likely to occur.
I am in favor of more gun regulation but it's not like if guns were allowed at a convention the position would automatically be proven right. There's a small chance that something would happen (which is reason enough to prevent guns in a large public gathering) but a much larger chance that nothing does, giving an arrow to the quiver of those against greater gun control.
I definitely don't think guns at a convention is a good idea due to the very small chance of a very bad thing happening but the likelihood of the danger being overstated really fuels a lot of the opposition to it and if gun control advocates make a big deal out of it and then something like this happens without any problems it doesn't help heir position any.
And because the venues have the rules not the party anyway, it just seems like a silly way to argue for gun control (by arguing against it for a single event). Their political counterparts aren't in charge of that decision.