Yes, so what happened before it was made illegal
And why accept european migrants but bar south american migrants.
There were many differences. One main difference was the Europeans had to cross an ocean. This meant their move to the US was a more permanent decision. This decision required the will to adapt and the need to assimilate. Most of the Europeans came over before the welfare state, which meant they understood they had to sink or swim. They did not have the option to recline in a social hammock.
Many, but not all Central American immigrants, come here for jobs, for free education and for free medical care. Those who work, still send money back to their native counties, and save for their retirement, in their cheaper countries of origin. America is a stop over place. This was not an option for the Europeans, until the welfare state was established.
The modern welfare state lure is why Trump wants only immigrants who can contribute; net give, instead of net take. This is more in line with the practical constraints of European immigration.
One way to do this is to stop welfare and all freebies for illegal immigrants. This will change the lure dynamics. The self reliant, who know they will need to work and net contribute, will still come. The first test of self reliance is the ability to do it by the book, instead as a thief in the night.
Current estimates are that the cost of illegal immigration is about $125 Billion per year. This covers everything from welfare, free education (local taxes) and free medical care. This a huge freebie lure that draws not only the poor, but also the con artists and criminals. Sanctuary cities will protect criminals since they are all blended with the poor and needy.
Trump wants to build the wall, which is a one time cost of about $25 billion with a $5 billion annual cost. This saves over $100 billion the first year, and then forever, yet politicians from both sides, complain. The question becomes, how much of this extra $100 billion in welfare cost is skimmed, as a campaign kick back to these politicians? All the middle men benefit, and they are all beholden to the makers of laws. If you run a shuttle service company for illegals, that the state pays, this makes you money. Which palm do you grease?
For example, free education for illegals is very expensive. The estimate is about $45 Billon. Although this is expensive to the tax payer, it benefits the teacher's unions. Having to teach all extra subjects, in fives or more languages, creates lot of new jobs for teachers and/or can be used in collective bargaining for a pay raise. How much of a kickback is this worth from the unions? Is it 1% or 5%. And who makes the most from this kickback? Why would they want to change anything? Why not expand it?
How about businesses, that use illegal immigrants for labor, but do not pay any formal benefits. The illegal immigrants will still get benefit from their state and the federal government, to the tune of $125 billion per year. In essence, the tax payer pays the labor benefits for these businesses. How much of a kickback is this worth?
The swamp is ripping off the country using illegals as pawns. Maybe we need to prohibit any politician from receiving campaign contributions, from anyone who is a middle man, for any goods and services, paid for by the tax payers, to illegal aliens. Without the skim, the hypocrites will change the system.