They came mainly from Syria, I believe. And they got a big population because Syria wouldn't let them come home.
Still won't.
the Palestinians (And the very definition of the word) are castaways from society. They are lower than the slaves of Syria and Jordan.
Palestine was a wasteland that they were sent to to die. And to this day, no arab aid comes to them.
Because they were supposed to die.
The desert there did NOT have a single mosque or Islamist influence because it was their waste land. This is why the Jews were sent there as well - as a sort of poke in the eye. But I can cover that debate at another time.