Yeah like Aimee said they are not an accepted color so you don't see them as much. They are wicked cute tho

They also have to have the required white markings to be accepted lol.
This is one reason I like Pit Bulls

They are recognized in so many colors and patterns. Danes have very strict colors as well.
Bostons came down from APBTs so they do carry the color. Its not a factor of mixing. Its and issue of what the AKC would prefer the dogs to resemble.