Yeah... that's a chart of linguistic diversity, which isn't the same diversity I am referring to. The diversity I am referring to is aggregate diversity. Different groups have different customs, different lifestyles, and different opinions on how daily life should be. Something that is acceptable here, may not be acceptable elsewhere.
For what ever reason, we have a huge problem with getting people to agree on things. Ever. Because of that, the US tends to have problems creating programs to benefit everyone. No matter what we do, someone always will be slighted or at the very least benefit less than someone else. When that happens, programs get stalled and fail. If we suddenly said, "Congrats, free education!" Many people would be elated (myself included). However, there's a lot of people with big money that might get taxed. They threaten to pull funding for something else and now those people rally against free education. Commence snowball effect.
When considering aggregate diversity, it is very hard to choose your metrics. It can be places of descent or ethnicity, but it can also be territorial within the boarders. I am originally from the north east US, and I moved to the south when I was about 18. There was a massive culture shock. People just did things differently than what I was accustomed to. There is no right or wrong, but there is often a difference in opinion. In NY, guns are a big issue. They (typically, but not always) feel guns are dangerous and want more restrictions on them. In Texas there is also a big issue with guns. They (typically, but not always) want guns to be owned and used properly, but not overly restricted. How do you pass a law that satisfies both groups? Many of us believe that it is a state issue, others believe it's federal. Now add in the fact that the groups are 2000 miles away from each other and have no idea what the other place is like. That's a diversity issue.
I'm not saying anything is better than anything else. Not at all. I am just trying to illustrate an issue that tends to cause us to not progress in the same direction as other nations.