The big difference today is that "diversity" didn't have a price when we were mostly agrarian, and had a frontier where people could relocate when they felt like it was time to move on.
Several things changed this dynamic completely, and they aren't all recent developments. In rough chronological order, I'll list a few of them here:
1. industrialization
2. the admission of Arizona and New Mexico into the Union just before World War I, which completed the contiguous 48 states and put everyone in this land mass under the control of one government
3. urbanization, and continued densification of urban areas
4. advances in telecommunications and computer technology, which effectively shrank the world and added a gray area in the definition of a national border
The end result of all this is that not only do we have a nation of 310-320 million people -- but we are living increasingly in circumstances where we have no choice but to interact with other people in the course of our lives.
This is why I believe that the core of most political, economic and social ills we face is that we are forced to interact with people -- under something other than our own terms -- who have absolutely nothing in common with us. And I don't just mean that in racial/ethnic terms, either.
The story of the Tower of Babel is in God’s Word for a reason. The “powers” are trying to rebuild the tower after God said “you can’t build that”, and scattered them into diverse tribes and different languages. He knew we accomplished more and remained better people in small groups with a common culture.
Why is it that we can’t all just get along? The truth of the matter is that we (the human race) just aren't as civilized as we would like to think we are. Our species, Homo sapiens sapiens, isn’t that long “out of the trees” and the veneer of civilization is very thin, to non-existent, upon us all. We’re all carrying the genes that got us here and some of those genes carry a very basic survival instinct that has allowed us to survive thus far.
This most basic of all survival instincts is self preservation and it’s the one that warns us to be very wary of those who are different than us, our family, and our tribe. Those who possessed this survival instinct survived and reproduced, and those who had no fear/distrust of strangers didn’t survive. Over the years, this survival instinct has been referred to as tribalism, nationalism, nativism, ethnocentrism, racism, and various other isms, but it’s an inborn instinct carried in our genes whether we like it or not.