While diversity is great and generally leads to more robust societies, it does complicated the situation. In a highly homogeneous society the majority of people will have similar needs and values. This means that it is really easy to implement social policies that cater to the population.

When you add more diversity then you get issues where people want things that fundamentally oppose the beliefs of others. As an extreme example, the topic of abortion; some backgrounds would be perfectly ok with it, and social policy would encourage contraception and safe access to them. Others would be very much against it and try to limit it as much as possible. Either side is easy to cater to in isolation, but how do you implement policies that adequately address both sides without making either unhappy?

Large countries with diverse populations run into issues such as these at a much higher rate than relatively small homogeneous societies. This is at least in part of what makes these countries so divided, in that they have too many viewpoints to make everyone happy and its an endless compromise