Why then did so few ever bother to make the trip? The answer is because they were already in their respective homelands: England, France, The Netherlands, Germany, Poland, Russia, Morocco, Yemen, etc, etc. following their religion in their native homelands.
That was already the case 2000 years ago.
How could so many merchants and money lenders survive in Palestine?
The ancient Israelite society was an agricultural society.
Palestina could never feed more than about 300 000 people.
In the Roman Empire there were a couple of million Jews, most of them lived in the ancient Megacities, like Alexandria, Rome, etc.
And Jews who lived outside of Palestine were not expelled from Palestine, we are talking about ancient converts to Judaism. So Alexandrian Jews were Native Alexandrians. Persian Jews were native Persians. Roman Jews were native Romans.
Judaism rejected agriculture as something that Jews should never do.
Jews did not have any peasants 2000 years ago, they could feed themselves only via selling things and lending money.
The Romans did not expel any Jews, with the exception of a few Zealots, speak ancient terrorists.
Diaspora was a logic consequence of Judaism. Jews were selling things, lending money, etc,, and leading a semi-nomadic lifestyle already 2000 years ago, and that continued till the 20th century.