How has it proven that? What I find interesting is that early on, our immigrants were predominantly from developed nations. Over time, this shifted more and more to developing nations. To me that seems to indicate that we're not seen as the land of opportunity among first world nations any longer. There are better options with more economic mobility and social services. Unless gun ownership is your primary motivating factor for immigration, there are many places where the working poor are better off going.
Both of these theories seem too simplistic. From the visualization, it didn't seem like immigration from Europe necessarily decreased in the late 20th century. It's just that the proportion from the rest of the world increased.
Also, countries like Ireland, Germany, and Italy weren't really "developed" prior to the 20th century (and to be fair, neither was the US). Germany and Italy weren't even united countries until late in the 19th century and Ireland was a British possession until the early 20th. It's possible Europeans are just staying where they are because conditions in their home countries have improved. Or, more likely, they are emigrating to richer EU countries because the Schengen agreement makes this much easier. Another factor is that a big draw for European immigrants in the 19th century was the large amount of unclaimed farmland out west. This is no longer the case now.
As for immigration from non-Western countries, they are coming over in larger numbers now because they are actually allowed to. Prior to 1965, quotas heavily favored Western European countries over the rest of the world[1].
