Why has America been referred to as " the land of opportunity"?

Same as with Canada which is where both my grandfathers came from. Let's see how many reasons I can come up with just off the top of my head and just for those two.

They enjoyed the freedom of the First Amendment (speech, press, religion, assembly - - Canada has the same provision) that was not granted in the country they left. They never exercised those rights, I don't think, but their children and grandchildren did. They were free to raise their children so that they had the chance of being productive. My father was an MD, but he owed that piece of good fortune to his father. The country from which they came would never have allowed him to get all that education. They were able to eventually bring their wives and children with them. There was enough money to be made, even at jobs that didn't pay much, to bring them across the Atlantic. They were able, once the families were here, to turn their attention to bettering their conditions. They never became rich, but no one starved either. That's more than could be said about those relatives who didn't do as they did. They were free to travel. They didn't do that, but their children and especially their grandchildren did. That too was very limited where they came from. They had medical care and good medical care which was not given to just anyone where they came from.

It provides anyone from any background the possibility of success through hard work.