I was casually looking at a world map and noticed just how close to Greenland Iceland is. I am aware that there were Viking travels to Greenland and what is now Eastern Canada during that age, but was this knowledge somehow lost? It seems remarkable that when Columbus sailed to try and find another way to the Indies that he had no idea that there was land where the Americas are after other Europeans had already been there and its not too far from Iceland and North West Europe.
The Greenland Norse were fairly isolated from Europe. They eventually lost contact with Europe, and it isn't even clear what the exact date of their disappearance was. So whatever knowledge the Greenland Vikings had about North America, including their explorations and encampments there, wasn't necessarily reaching other Europeans.
The Greenland Vikings didn't necessarily recognize that there was a major landmass to the west. We know they visited the island of Newfoundland and set up a camp there. Recent evidence suggests that they also had a settlement or camp on one of the Canadian Arctic islands. But those are islands, not the mainland. Even if they did see or visit the mainland of North America, there's no evidence that they even realized that this was a major landmass and not just another island.
So, basically the Greenland Vikings had little contact with Europe, so whatever they knew about North America didn't necessarily reach Europe. Or it may have reached Europe in semi-mythic form, so that it was unclear whether it was accurate or not. Secondly, it isn't clear that the Greenland Vikings even realized that they had reached an entirely new continent. They may have thought that they had simply found some more North Atlantic islands like Iceland, Greenland, etc.
When Columbus sailed west he took a more southerly route than the Vikings. Even if he knew about the lands "discovered" by the Vikings in North America, he would've had no reason to think that this landmass would extend all the way south to where he was sailing.