So, former Catholic and amateur theologian, I can kinda explain the way this works.
Basically, when it comes to the RCC, everything is about tradition. The bread and wine need to be prepared a certain way. A fun factoid: during prohibition, Catholics were still allowed to make wine because, literally, they had to for their religion. It's part of the way things are done, so that is how they are done. The bread has specific ingredients that haven't been changed in forever, and that's just how it is. It also has to do with the nature of the Eucharist with the RCC. It's the focal point of the religion. Think of it like how most Protestants view the Bible: you can't change it, because it's that important.
I do remember a priest saying that if you have a gluten allergy, you can just drink the wine. If you have an allergy to both...you're kinda SOL.