The best answer would be this: because Polynesian colonizations even to Remote Oceania proved these peoples’ impressive seafaring capabilities, whereas there are no comparable records for Atlantic voyaging. That good scientific reason isn’t all the story. The myth of Columbus focuses on the Atlantic, the theater of that Admiral’s exploits and suffering, portrayed as bordered on the east by progressive civilizations and on the west by savages in wildernesses. Had there been prior contacts, the supposition goes, America would not have been such a wilderness-see how quickly it was cleared after Europeans landed! Related to that notion is that contact with Europeans infected American natives with devastating epidemics of Eurasian diseases; if no evidence of such devastation is recognized, it means no contacts. “Knowing” this, evidence suggesting pre-Columbian epidemics is attributed to indigenous pathogens. Power of chartering myth is especially apparent when it hits so close to home.