Because when your whole campaign is based on Terror of the Other, then even people who come here legally are fodder for stereotypes, fear-mongering, and discrimination.
Legal immigration policy is one of those things that isn't quite as clear-cut as to when it's bad and when it's good. Aside from some fringe open borders types, most people agree that there should be some sort of restrictions on numbers and criteria for people to come here legally, as, in fact, there are.
From there, though, the discussion goes a bit wonky. Do legal immigrants push down wages? While you can probably find anecdata to support that proposition, the overall economic judgment is no.
Is America being overwhelmed by People Not Like Us? That's a subjective assessment at best, thinly masked racism at worst (it presupposes that there is a singular "like us" to compare to), but the same accusation has been leveled in every era, and we generally look back on those accusations and roll our eyes at the stupidity.
Does it make sense to encourage people coming here who already speak English? The advantages are obvious (until we get nativists complaining about people using "-our" instead of "-or" and "s" instead of "z"), but the way that tilts the racial and ethnic balance needs to be looked at, too, so that this doesn't become a Poll Tax equivalent.
Family create stability in society, as the Republicans usually argue vociferously. Does that mean it therefore makes since to restrict further who gets a leg up on coming into the US based on family already here? Hard to see that.
And, of course, there's the particularly naked hot button here, preventing legal, unnaturalized immigrants from receiving any sort of federal welfare. The arguments behind that are obvious, to prevent the moral hazard of people coming here just to get on welfare programs. On the other hand, it's unclear that this is significant problem (some numbers might be nice), and, honestly, circumstances change — that great job that you gave up your life to accept here suddenly goes away, and now you're faced with either starving or trying to finance going back home again? Do the people writing this bill think it's that easy to either move here or to leave? There are a thousand stories of that sort that can and will be told, and how they will play vs. the "Poor furriners are coming to take your tax money!" meme will be interesting to see.
Given some of the folk pushing this bill forward, it's not unwarranted to suggest this is not a good faith effort to solve an agreed-upon problem, but a modern Know-Nothing movement out to protect White Christians from Them Folk What Ain't. And for many Americans, it calls to mind where we might be if someone had said "Only wealthy or gainfully employed people who can speak English are preferred to come here" when our own ancestors were "yearning to breathe free." I, for one, would be writing these words, and the face of the nation today, and what would be lacking that we received from poor immigrants who couldn't speak a word of English, would be very different indeed — and not for the better.
Trump Unveils Legislation Limiting Legal Immigration
There has been a huge debate about immigrants who are in the U.S. illegally, but the administration is looking to cut the number of legal immigrants allowed into the country by half.
I'll add as an additional comment that a lot of people are considering this bill DOA, that between people who think immigration brings new ideas and new vigor to our country, and people whose business interests depend on green card immigration, that the Senate will be in no mood to pass it. And they may be right.
That said, the purpose of this bill may simply be, if not to distract from other failures and scandals in the Trump Administration, to open up another rhetorical war front (or expand the current one against Those Damn Furriners) as a cultural/political war that will play out in the voting booths in a year-and-change. It's an attempt to fulfill a campaign promise to nativists and rally them to Trump's and the GOP's cause. And it adds to the sense of Fear both to discourage people from coming here, and encouraging folk here to worry about what Outsider atrocity will happen to them and their loved ones.
Whether the bill passes or not, that emotional impact around the debate will exist.