The answer to this can be found in Chapter 10.
There, Diamond tells us that there are two things that make diffusion difficult in the Americas. First, there is the fact that the continents' long axis runs north to south. This makes it difficult for plants to spread because the plants would have to spread across different latitudes and different latitudes tend to have different climates. Second, there are natural blockages in the Americas that make it hard for a plant to spread. These include the Isthmus of Panama (jungles) and the deserts of Northern Mexico.