Which pretty much sums it up. Once the immigrants had sufficient numbers to aquire a national identity, you can't just disperse them again. The same thing goes for the people who used to live there and were forced from their homes, once they were in sufficient numbers, they also gained a sort of identity as a group.
There isn't really such a thing as a palestinian "nation", but people who used to live in the space between Jordan, Lebanon, Egypt and the Mediterranean are united by a common cause, hatred against those who occupy their ancestral land and the prospect of not having a future.