Well one reason is probably because breading different types of Panther's is still breading within the same species.
Your talking about breading different species together. It is possible with some very closely related species and in fact there's a recent thread about that on this forum. The feelings about that are rather mixed. Generally though most chams of different species can mate but not produce viable offspring. Just for fun let's see a Meller's and maybe some pygmy species.