Lucky, it's probably because they figured that Red and Leif were just pesky scandanavians looking for trouble...the Vikings of old kind of gave themselves a bad rap with that "looting pillaging" thing and the Ericsons were cought up in that stigma...
Seriously, they have not only played down the fact that there is well documented proof of at least one Norse settlement in North American soil, both in the actual ruins, and written records, but they also do everything they can to disprove that Norsemen ever sailed past Iceland.
There is even possabilities that Norsemen made it as far as Mexico. Experts have said that Montezuma thought that Cortez was the return of the "visitors" who came centuries before him. Not wanting to anger what Montezuma thought were dieties, he held his massive military in check.
The old legend referred to the visitors as having fair skin, some had golden hair or flaming hair and all had shining skin. Either the Aztecs got into a bad batch of liquor, or they were looking at Europeans...
If it wasn't for that legend, the Aztecs would have snuffed the Spaniards before they ever got as far as they did in Central America