Durability and cleanliness. Fabric seats do not age well and are hard to clean. Real leather ages extremely well, and is easy to wipe clean. Fake leather or semi-fake leather (e.g. bonded) are easy to clean but age extremely poorly.

I find some nicer fabrics, such as Alcantara, to be more comfortable than leather, but they soak up oils and can end up looking gross as they age.