The arabs who left during 1948 war were forced out of their homes,they didn't sell their land.
This is the Palestinian propaganda version but it is not what happened. Very few Arabs were forced out and those that were forced out tended to be militiamen. In most cases they left before Haganah captured their areas, and in most cases some stayed and some left. Those that stayed were well treated, remained in their homes, and form the modern Israeli Arab population.
The reason why they chose to leave is debated by historians and while there is no agreement the most commonly discussed factors are:
1. The Arab League asserted that any Arab who accepted Jewish rule was a traitor
2. The Arab side spread propaganda that any village that fell to Haganah faced slaughter and rape
3. The invading Arab nations asked them to leave to clear Muslim civilians out of the path of a what they believed would be a quick and decisive defeat of Jewish militias.
4. Jewish terrorists like Irgun really had massacred people at Deir Yassin
5. Jewish extremists also spread propaganda to Arabs about Deir Yassin
So some people on both sides were spreading false propaganda that massacres were common when in fact Deir Yassin was exceptional (in several places Haganah fought against Irgun to defend the Arab populations and in the end Haganah, which became IDF, disarmed Irgun at gunpoint with a surrender or die ultimatum).
But false propaganda was received by Arabs from extremists on both sides, together with Arab league threats that accepting Jewish rule was collaborating, and with demands from other Arabs that they leave. In addition to that, the ruling class of Arabs, the wealthy, had exited the conflict areas early on for safer homes in Europe and elsewhere in the Middle East.
Add on that the country was engulfed in a civil war that had all the sorts of collateral damage any war has, bullets and mortars flying, and in this context many left, but many also stayed and those who stayed were well treated and made equal citizens.
In any case, they weren't forced to leave and this who chose to stay weren't mistreated.
And you're talking about immigration. The israeli governement call it right of return not immigration. So if the right of return applies to let's assume the descendants of those who left 2000 years ago ( not arguing whether they are their descendants or not , this is another topic) ,it should apply to the descendants of the ones who left 66 years ago.
In reality it i not a right of return, there is a clause in the Israeli immigration law that creates an immigration category for anyone who is Jewish. I would point out that many nations give preference to immigrants of their own cultural or linguistic background, it is common around the world.
The actual right of return in the UN Declaration of Human Rights entitles people to enter their own country, it is not a mechanism that grants citizenship to a country you never lived in.
Though if you read Palestinian propaganda that fictionalize it into some inherited citizenship. A reading of the actual Declaration however clears that misconception up.
