The Road Not Taken is a great example of structured, lyric poetry. Some would also put this poem, and others like it, in the category of "nature poetry." It is also an example of modern poetry in the broadest sense.
Frost was born in the late 1800s and died in 1963, but he was not part of the literary modernist movement that defined many of his peers. His poetry emphasized place, rural life, nature, order, and an abiding view of life as unchanging and unaffected by societal progress.
Modernists, such as Langston Hughes and e.e. cummings, experimented with form, topic, and perspective in ways that Frost did not.
The genre of nature poetry has persisted over centuries. In some ways, the romantics delved into harmony with nature, as far back as the mid-1700s. Modern poets with this bent include A.E. Houseman, John Keats, and Gerard Manley Hopkins - each of whom came from a different era and who had dissimilar styles.
Frost's poetry uses tight, direct structure and natural metaphor to create both a scene and the framework of an idea, yet the universality of his perspective is welcoming to the read. He sets up a proposition that the reader solves, drawing the reading to the setting and the idea. Thus, nature is the perfect vehicle, since its universal metaphors are both familiar and appealing.