Culinary tradition is not always based on fact. Sometimes it's based on history, on habits that come out of a time when kitchens were fueled by charcoal.
Alton BrownButter typically melts around, you know, 90 Fahrenheit, as opposed to about 20 degrees higher for lard. So what that means is that when you're working with it, the lard is going to stay more solid, which is great for flakiness.
Alton Brown