Because Vikings Just Luuurved to Travel
You've got these guys, right? They're mostly farmers, but they do a little sailing and trading, and sometimes even a bit of raiding, depending on what the local economy will stand. They set out in a tiny ship, navigating west with a tool which is basically an advanced sundial, and which doesn't do well unless you want to go in an east-west line. What the guys are looking for is land. Silver is nice and gold is better, but what they really need is somewhere to put their cows.
They pass Iceland and Greenland, which are both well-settled by that point, and keep on going. After weeks at sea--LAND! It's cold and damp and dark, just like home! Cool! Let's settle here! ...naaah, let's go a little farther south. Oh, look! more land! It's still chilly, but it looks fertile. How 'bout here? ...naaah, a little more. Okay, it's getting warmer now, and the land is VERY fertile. We've been gone a couple of months. How 'bout here? ...naaaah, a little farther. So they pass land which is increasingly fertile, then downright lush, then practically voracious (shove a dry stick in the ground and it'll bud), then they go 'round what turns out to be a big bend and pass more blisteringly hot and lush land, then slightly less blisteringly hot and lush land. Now they've been out for at least five months, probably more. Their families need them desperately at home. They've been eating nothing but what they can catch and gather. How 'bout here? ...naaaah, let's go up that big river. So they go upriver. Deep black soil to either side. It starts getting cooler. How 'bout here? ...naaaah, just a little more.
Finally they come to a flat, rich land, well-watered, perfect for cows. Here! Great! They all pile off and some of them start farms while the rest turn around and sail back several thousand miles for their wives, children, servants, slaves, and cows. The second trip out is kind of rough. The wives, children, servants, and slaves all want to know why they're not stopping in the cold fertile land, or in the cool fertile land, or in the warm fertile land, and by the time they're reached the hot lush land even the cows are starting to look puzzled. And then they go 'round the bend and start up the river, and there's practically a mutiny. They finally reach the place where the crew have been setting up shop, and everyone has to admit that it's good cow pasture, but still--sweetheart, why didn't you get off the ship two thousand miles earlier?
The sailors have to hear that every time they go out to get another boatload of colonists.
Finally, they say, "Hell with it, let's go home. The beer's bad here and the locals are looking at us funny." So they pack up their goods and take down the houses, being careful to uproot the house posts, fill them in, and shovel all of their hearth ashes into the river on their way out. However, they do decide to leave a couple of carved stones behind. There's no room for them on the ship.
Hundreds of years later, a new wave of colonists finds the stones. They immediately realize that ohmigod, there were Vikings in Minnesota!