Because it's a cat. Darwin explained that, rather clearly, when he published an explanation of the theory. It's available in lots of good libraries, so you could save yourself a lot of wasted time by studying it. Do the homework!
Felines and canines do share common ancestry. But that was many generations ago, and since that time felines and canines have evolved and diversified separately.
Of course, the domesticated ones have diversified even more. Darwin started out "The Origin of Species" by pointing out how quickly breeds of domestic animals (or plants) can be developed by human breeders. His point was that what he and Wallace called "natural selection' was a similar process to domestic selection, but slower and driven not by intelligence, but rather by success at survival.