It’s enough for the world to be multicultural. That’s a good thing, and makes life more interesting. It’s not necessary for nations to be multicultural, and unless there is something about a nation that all citizens are proud of and can coalesce around, multiculturalism will likely split that nation.
I use the analogy of a neighborhood, or your street.
Your street has houses, each with a single family living within that house. Your street can have families from all kinds of cultures living on it, in harmony. That’s because inside of each house, each family can live the way they wish, and use their resources as they wish. If you are a guest in their house, you respect the rules of the house, and you expect any guests to your house to do likewise. A nation, therefore, is a house, on a street full of houses.
What muticulturalism is is putting multiple families with different cultures under the same roof, sharing the resources. You can easily see where conflicts will arise, it doesn’t mean one family is better than the other, but it’s just human nature to want to control your surroundings.