I think there are a few things behind this: 1. price. Carbon fiber is a lot more expensive than aluminum 2. The wings are the parts that need the most strengt

I think there are a few things behind this: 1. price. Carbon fiber is a lot more expensive than aluminum 2. The wings are the parts that need the most strength, and where the flexibility of design helps the most. 3. Making a giant tube of aluminum is a lot easier than with carbon fiber, but the complex shape of a wing evens the cost more 4. Carbon fiber advantages shrink with larger aircraft since making the ovens for really big carbon fiber parts is ridiculously expensive. I wouldn't be surprised if smaller planes go to all carbon fiber before bigger ones.