Because they mix well and make optimal use of the paddock space that way.
Why does everyone always react in horror when species from different continents are mixed? Maybe it's better (education-wise) to only mix sympatric species but how specific does it need to be? Grevy's Zebra and Scimitar-horned Oryx are from completely different areas of Africa and would never meet in the wild, but because they're from the same continent no-one complains (or notices?). Sorry to pick on you in particular Ciaran, as you're far from being the only one, but it's something that always slightly bugs me.
