People who hate or otherwise have a strong dislike for their country:
Hi.
Why should people come to your country?
Because they have been attacked by, or by armed forces funded through, a state power necessarily supported through coersion of its native populace (read: taxes). Understandably they are upset, and they have been offered a piece of welfare-pie in exchange for voting to expand state power should they immigrate. Almost everyone living in the crapsack of the Third World benefits from coming to the First World, and the incentive to do so is stronger now than ever.
I don't think this really answers why they should come here, and I'm unsure whether that is even a meaningful question, but it is at least why they do.
What makes it great?
An imperial legacy that improved the living standards of millions across the world, the crowning achievements being the ending of slavery in the West, and the simultaneous defeat of widespread piracy.
This was in the past, the slow updraft after the collapse of the Roman Empire; now we're on precipice of the next decline.