Aero. Obviously nobody makes any high-performance aero road racing rims that are designed for them, but wider tires tend to increase CdA in general. Whether it's actually significant is a matter of perspective, since even a 2" tire is a pretty tiny object compared with, say, a cyclist's torso.

The particular tire used tends to be a lot more significant than the size. Riding on the flats in calm conditions, I don't really notice a difference between my gravel bike with its 53mm Rat Trap Pass ELs and my skinny-tired road bikes; but when I had ThickSlicks on the same bike, it was averaging a pretty considerable ~5% slower in the same circumstances.