If you are asking what characteristics of the plants made them "better," the main answer is the size of the plants' seeds. Diamond talks about how many of the plants in the Fertile Crescent were among the wild plants that had the biggest seeds. Plants with big seeds are better because the seeds are the things that people eat (if we are talking about grains). You can find this in Chapter 8.
If you are asking why Eurasia had the plants rather than some other place, the answer is luck. One thing Diamond is trying to argue in this book is that some places were just luckier than others in terms of plants and animals that were available to them and in terms of their geography.