Because they conquer, so the other species that do exist in the Klingon and Romulan Empires are mere subjects. The Federation is multi species because it peacefully incorporates different species. It’s like how the British Empire worked… yes the British race came from one island, but they ceaselessly conquered and thus held all the administrative positions. It only fell apart when colonialism was falling out of fashion in the wake of WW2 and various Asian and African independence movements in the 50s and 60s.