All of these countries were essentially homogeneous or had homogeneous authority during their rise.
Also empires usually have absolute authority, you cannot play ethnic cards with that authority and expect to carry on. That's one of the reasons empires collapse with time due to growing dissent in different parts of the empire.
Good point. India is heading towards (or has already reached some level of) authoritarianism and it is not just my opinion. Multiple articles on this available. Thanks for the indirect agreement.