What is the most likely reason that thoreau structured this text around the idea of a wild apple tree as a noble fruit

Thoreau called the wild apple tree a noble tree because it symbolizes knowledge.

Explanation:

Thoreau was one of the most imminent transcendental thinkers of the mid 19th century. He believed that  spiritual unity could be achieved by knowing and then feeling oneself to be one with nature, the path went through knowledge.

As such, the tree of apple has a biblical allusion and represents the tree of knowledge that Adam eats from. The same tree can be said to be noble if the person eating from it can become one with nature with that knowledge.