Here's why:
* Sonnets have a specific structure: Sonnets are poems with 14 lines written in iambic pentameter (a rhythm pattern of 10 syllables per line). They also follow a specific rhyme scheme, either Shakespearean or Petrarchan.
* "The Road Not Taken" doesn't meet these criteria: It has only 16 lines and doesn't follow a strict iambic pentameter or rhyme scheme.
While the poem is a classic and has a memorable structure, it doesn't fit the formal definition of a sonnet.