Because Cats > All.

That and people were asking for a more "Beastly" race to be playable. Thus, the Hrothgar, based loosely on Ronso. Added with multiple head types (One of them being cat-like)

That's the main difference between Hrothgar and Miqo'te - It's less about them both being cats and more that Miqo'te are kind of like people with cat features while Hrothgar are more anthropomorphic cats/wolves/whatever other heads they have to choose from.

Yoshi thought about adding the Lupin as the "Beastly" race, but didn't feel it worked that well and so ended up making the Hrothgar instead.