Why does aristotle claim that the republic that socrates sketched it too unified?

You try to tell people that they are naturally not cut out for ruling and must forever herd sheep. People don't just naturally accept their position in life: "all claim to possess political ability, and think that they are quite competent to fill most offices. But the same persons cannot be rich and poor at the same time. For this reason the rich and the poor are regarded in an especial sense as parts of a state. Again, because the rich are generally few in number, while the poor are many, they appear to be antagonistic, and as the one or the other prevails they form the government." -Book 4 Politics.