Essentially, the pigment which causes brown skin and brown eyes is just darker than other pigments, so regardless of whatever other pigments are present, brown is the one that's going to be visible.
However, there's as much variation within other races, but they vary in different ways - for example, people from one race might all have generally similar eyes but different noses, while people from another race might all have different eyes but generally similar noses.
The way the brain recognizes faces is highly specialized (we're much, much better at recognizing other people than we are at recognizing individual sheep, for example), and works by focusing on the parts that are most variable.
If you don't grow up around Asian people, you might find "all Asians look alike", because your brain doesn't know what to look for. If you do grow up around Asian people, you're less likely to have that problem, even if you are not Asian yourself.
Since blue eyes and blond hair is primarily a Caucasian mutation, it gives us the opportunity to create many variations, such as green eyes, as a result. As for the other races you've mentioned, they are Earth's default. All humans originally had dark hair and dark eyes.
Not entirley sure, but I'm sure you know orgininally humans are from africa and then populated other regions, developing different physical features to deal with the different environments.
What is Caucasian really? It pretty much means European, at least what most people define it as would be. When humans imgrated from africa to Europe, their bodies needed to change to survive in the harsh climate much more than they needed in latin America or southeast asia.
The southern parts of Europe had been way more populated than the north for centuries and when southern nations occupied the north, they mixed with people there. And that has been happend for centuries too, while people in africa, latin America, asia or even north America mixed more with people genetically close (until European occupied those areas that is). My point is thousands of years ago Africans immgrated to Europe and asia, and it didn't happen the other way around until a few hundred years ago
Quite frankly white people have dna that isn't dominant when mixed with other races. Mixing dark and white skin or blond and black hair is the same as mixing black and white paint. The darker will be more dominant