The answer to this can be found in Chapter 18. Specifically, it can be found on pages 370 through 374 of the paperback edition of the book. As is most often the case in this book, the answer to this question has to do with geography. Diamond argues that the Norse, like Leif Ericson, failed because they tried to settle areas with bad geography whereas Columbus came to a much better area. He also believes that the Norse were the “wrong” people to attempt to conquer. As we see on p. 373, Diamond believes that
The second Eurasian attempt to colonize the Americas succeeded because it involved a source, target, latitude, and time, that allowed Europe’s potential advantages to be exerted effectively.
The Norse came to a bad area. They came to Greenland and other northern areas that could not really support farming. This made it very hard for the Norse to sustain their colonies. In addition, the Norse came from a population that was too small and too poor to give the settlers enough help.
By contrast, the Spanish came to subtropical areas that were “highly suitable for food production.” They came from a relatively rich and populous country. Finally, they came at a time when Europe was more technologically advanced and had the ships and such that were needed to support the settlements.
For these reasons, the Norse failed and Columbus succeeded.