I think that should be fairly obvious - money + ease of traveling to. America is, relative to the world, perceived as quite wealthy. South America is full of places that are quite poor. Put the two side by side and many guys coming here speaking not a lick of English, and with no skills to boot, probably envision themselves coming home rich.
It's even relatively easy to put yourselves in their shoes. Columbia's GDP/capita is about $8k. In the US it's about $80k. Imagine how you'd feel if Canada had a GDP/capita of $800k. To many people it'd seem like a great idea to move there completely regardless of everything else about the country. People warning you that you'll end up mowing yards and painting houses while making barely enough to put a roof over your head. Bah! Nonsense! How can that be true on $800k/year!? Canada, here I come!
You can see this play out the same in places like Saudi Arabia. Not many place have the taste for their policies, religion, or much of anything else - yet they have a massive immigrant population, far higher than the US (as a percent) precisely because they pay stupidly high wages, often tax free, and have a low cost of living. You can easily become a dollar millionaire teaching English there if money is what you're after simply because you can easily save thousands of dollars a month. And if you get bored you can go watch somebody get crucified for witchcraft on a weekend now and again.