It’s because many cultures are incompatible. Once (via immigration) you let enough people of another culture into your own nation, it now becomes a battle to establish values. Immigration without assimilation will inevitably lead to disunity and decline.
The globalists are using immigration, largely muslims, to weaken nation states, over burden their economies and bring them to ruin. Welcome to the New World Order. I can’t believe Americans are voting for their own enslavement - but here we are.