One element of this episode that I find fascinating is the extent to which the Genoese chronicle may represent an isolated survival of a once much more widely propagated oral tradition. The suggestion that the author could learn of the existence of northeasternmost North America through sailors who partook in a body of knowledge common to European sailors that included knowledge of a land just beyond distant Greenland is fascinating. There may well be all sorts of things that people knew but just did not bother recording, whether because they thought it was not worth noting or because they thought that there was no need to do this since everyone knew it.
If there was more widespread knowledge of northeasternmost North America than we might have thought, though, this underlines the extent to which people did not care about the discovery. Helluland/Maryland/Vinland going almost entirely unnoticed by Europe makes sense, if you assume that it was a desirable resource-rich territory particularly accessible, only if you assume that hardly anyone knew about it. By the time that reliable chroniclers in northern Italy are commenting on the existence of the territory as a matter-of-fact aside, lots of people know that Helluland/Markland/Vinland exist.
Northeasternmost North America was overlooked not because no one knew about it; it was overlooked because it was not worth paying attention to. The Greenland Norse seem to have paid attention to the shores of the adjacent continent mainly because it was a useful place to scavenge resources, like wood and the walrus tusks that were key exports. For more distant population, this land located far from not just Christendom but the whole of the inhabited world with sparse resources and a hostile native population would have been a curiosity. Why bother colonizing this land?
That there was no sense, not apparently as reported by the Norse and not as recorded by Europeans, that these three lands were part of a larger continent is also key. If Helluland and Markland and Vinland were just marginally habitable lands scattered in the Arctic region of the world ocean, lacking in any geopolitical importance, what would be the point?
If you wanted to get European navigators to build upon Norse knowledge to do a Columbus and establish regular contact with the Americas, I think that the best way to do so would be to find local resources. The fisheries of the Grand Banks strike me as the most obvious possibility in the region of the Norse discoveries. As others have mentioned, English and Basque fishers may well have been fishing there—and using adjacent Newfoundland as a convenient base—even before Columbus. Could they have been found and exploited earlier still?