The previous posts were quite lucid in their explanation. I would suggest that there is a thematic reason as to why Wordsworth is a poet of nature, as well. Part of the driving force behind the Romantic thinkers, of which Wordsworth is an essential component, was to create a realm that was different than the preceding literary movement, the Neoclassicists. The Romantics wanted to conceive of a setting which was not entirely urban, did not focus on socializing with others, and develop an individual, as opposed to collective, sense of self. In attempting to tear away the mask of inauthenticity that dominated their perception of Neoclassicism, Romantic thinkers saw nature as the perfect setting for their ideas and beliefs. Its purity and splendor, its experience on an individual level, and its presence helped to fuse the duality of mind and heart. This appealed to Wordsworth, which is why so many of his poems have implications to the natural world or use it as their setting.