Because Humans are tribal by nature IMHO. We buy into it because Identity and loyalty to the group is powerful. There is also the tricky issue of negotiating a melting pot among unequal groups.
One thing I usually say about the Alt-Right and Ethno-States that they talk about is that it is mislabeled. The Alt-Right wants an explicitly RACIAL state. I.E. For them you must "be white," in both manners, customs, culture, and blood. The Alt-Lite, and the Civic Nationalists and likely even Conservatives like most of the GOP all the way to the Bill Kristol, Charles Krauthammer and some small sliver of Democrats and Liberals is IMHO not much different. Most Civic Nationalists types still insist on a national culture and language that is specific to one ethnicity of European, English people specifically. The problem of the "Lets come together and melting pot and become one people" is that in the end that usually means many people becoming like one especially powerful constituent part. There is still a latent racism within it, even if it can't be helped. Thus I'd say the Alt-Right are Racial Nationalists where nearly every Conservative and even many Liberals are Ethnic Nationalists. For them you can be whatever skin color of Human, but you still have to "Act White."
Identity politics will remain powerful because ....
A) Humans tend to schism into separate groups no matter what.
B) In a multicultural/ethnic/racial(?) differences and group allegiance will likely overcome other allegiances such as to class, gender, ect.
C) Even forming a "Melting Pot" comes with obvious undertones of racism, as is the case in America where assimilationist policies are more aptly cultural annihilation for some, into the culture and language of another.
Meritocracy IMHO will never fully function or work. It is a biological fact at this point that people will do whatever it takes to help give their kids an edge. Nepotism, and favoring your immediate kin over others, even if more qualified is just simple instinct.
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Democrats make the mistake of being clustered in two states.