To find an answer to this question, one place to look is in Chapter 8 of Guns, Germs, and Steel. In that chapter, Diamond discusses why agriculture never arose independently, or arose but only relatively late, in some areas of the world. As part of this discussion, he looks at what causes plants to be (or not to be) domesticated.
Some of the things that cause this are very obvious. As Diamond says on p. 133, plants can be rejected for domestication for obvious reasons like the fact that “they are woody, they produce no edible fruit, and their leaves and roots are also inedible.” In theory, then, you could submit this as your answer because it does give three reasons why people might not domesticate a given plant. However, if I were assigning this question, I would want you to look for a slightly less obvious set of answers.
In this chapter, Diamond discusses why the plants of the Fertile Crescent were so good for domestication. Let us look at what he says in order to find three factors that determine whether a plant will be domesticated. First, the size of a plant’s seed matters. People want to domesticate plants that will provide a lot of food. The bigger the seed (in a grain plant), the more food it provides. Second, he says that the way a plant grows matters. If a plant grows in “in large stands whose value must have been obvious to hunter-gatherers (136)” it is much more likely that people will notice it and try to domesticate it than it would be if very few specimens of the plant grew in widely scattered locations. Finally, Diamond says that annual plants are better for domestication than perennials. This is because these plants do not put their energy into growing anything like thick stalks that can survive from year to year. Instead, the bulk of their energy goes into producing things that are edible, making them much more useful to people.
These are some possible answers to this question. Other possible answers can be found in Chapter 7, for example, on pp. 123-4.
The first element underlying the choice is utility. The first plants to be domesticated were food crops. For people living in subsistence economies where life was a constant struggle to find food, there was no point in investing time in domesticating plants that were not edible. It is only after a society develops a food surplus that it can afford to expend resources domesticating ornamental plants for their beauty. We also now domesticate certain plants for soil stabilization, shade, and their wood, but these modern innovations depend on an economic infrastructure that did not exist in the periods Diamond describes. Instead, the favored plants for domestication were those that offered the greatest amount of high-quality food for the least amount of effort necessary to grow them.
The second element needed for a plant to be easily domesticated is that it has a large seed that can be easily gathered and planted. Self-pollinating plants are also preferable due to the ease of cultivation. Basically, as you will discover if you ever try to grow a few of your own vegetables, some plants are easier to cultivate, and most people sensibly prefer to choose plants that will thrive when cultivated. These are generally plants that are fairly hardy and able to handle an occasional dry spell, frost shock, or heat wave.
Finally, domesticated plants species tend to cluster in "packages" of plants that fill specific food niches and which can be cultivated together readily, such as the corn-beans-squash triad favored by the Anasazi people. Not only do the three plants together provide complete nutrition, they complemented one another in a system in which the corn was planted first and then created shade for the beans and squash. An ideal food package provided grains, legumes, and oils. Rather than cultivate a wide range of plants, most early societies tended to favor a few especially successful or useful plants to satisfy each niche.