Calfskins are not as much available as cowhides. For the simple reason that calfs are slaughtered a lot less then full grown cows. Cows are bred for milk and meat producten; a cowhide is always just a by-product. A slaughtered calf won't give a farmer as much profit as a slaughtered cow.
A lot of calfskins are very often stillborn animals or animals that have died in the first days or weeks after their birth.