The answer to this can be found in Chapter 7. Basically, some plant food sources have not been domesticated because they are not suited for domestication. Diamond emphasizes this point because he wants us to understand that we should not say that certain people are backwards because they did not domesticate plants in their areas. Instead, we should look at whether those plants were good for domestication or not.
Some plants, for example, have seeds that are too small to be easily harvested. Others have seeds that do not taste good. Still others have stems that break easily, making the seeds fall to the ground (which is fine for the plants, but not so good for someone trying to pick them).
The basic idea, then, is that some plants have yet to be domesticated because they are not suited for being grown and eaten by people.