It's not ASB in and of itself, as obviously IOTL Europeans were exploring the Americas earlier.
However, exploration in the 1100's is a lot harder than exploration in the 1500's. European maritime and navigational technology is very undeveloped. The greatest European sailors of the time, the vikings, had to island-hop to reach the Americas and even then they lost a horrendous amount of ships. If I remember correctly, of the 25 ships that left Iceland for the initial settlement of Greenland only 14 made it to the destination. So the voyage is long, arduous and dangerous, and goes through the 'choke point' of Greenland and Iceland as Europeans at this time could not make the direct voyage. Not only that, but the Medieval Warm period would end soon, so 1100-1200 is about the only window for European colonization before the weather makes the journey not worth it, even for the Norse.
In addition, there's not as much motivation for colonization. Trade with the Byzantines, the crusades, and even trade with the Mongols would give Europeans better access to spices and ivory in the Medieval Era than any purported westward passages to Asia. There's a much smaller population in Europe so less pressure for people to seek land elsewhere, and finally Islam is not seen as that much of a threat. Constantinople still stands strong, and the Muslim invasion of Iberia has petered out, so there's less Christian anxiety about any Muslim threat-and therefore less desire to seek out Prester John (the legendary Christian King in Asia or Africa, depending on who's telling the story) and new converts in faraway lands.