Firstly if people have large bags, they will needs to be searched and that will slow down people entering the site and cause queues and crush risks. Secondly, if the weather is that bad, likelihood is you'll be wearing rain gear going in anyway. It's quite common for venues to restrict the size of bags being brought into it. We're perhaps more familiar here with being able to bring the "hang sangwich" and the flask into Semple Stadium but places like the Bayern Munich and Fenway Park in Boston have luggage parks where if you have a big bag, you'll be told to leave it there in a locker or bag park outside of the ground