Answer:
Because It has famous quotes in it. :)
Explanation:
Firstly, Shakespeare's "Sonnet 18" is so famous because, outside of perhaps "to be or not to be," it has some of Shakespeare's most famous lines and phrases that most people have heard even without knowing the poem. For example "Shall I compare thee to a summer's day?" and "darling buds of May."
Secondly, is his answer to the question he poses in the first line of whether she deserves to be compared to a summer's day. His answer seems to be does summer deserve to be compared to the woman. Sometimes summer is too hot or windy. Other times the sun disappears behind the clouds. This woman, however, is always beautiful and will remain beautiful for the remainder of her life.
He immortalizes the woman through simple and clear but powerful language and metaphors that anybody can relate to. Summer, for example, isn't constant or reliable.
