Why were immigrants, especially, trapped in the pool of unskilled labor?

because they wherent taught much

Explanation:

People didn't respect others then.

Well, people don't respect others now but back then they didn't want people to "Steal Their Thunder" or something like that...

so people made sure they were left with the worst, never giving a chance for people to prove that they're good enough.

Its sad but thats how the world is, was and always will be, unless someone changes that.

But most immigrants back then couldn't even step up for themselves in fear of getting jailed, mistaken, or misunderstood so they just dealt with it.

the declaration states, “we hold these truths to be self-evident, that all men are created equal, that they are endowed by their creator with certain unalienable rights, that among these are life, liberty, and the pursuit of happiness….”