This is an opinion question, and so responses could vary widely; however, two arguments immediately come to mind. Romeo and Juliet is included on many high school syllabi because it is a classic text and the themes and ideas in it are still relatable.
Romeo and Juliet is part of the accepted core literary canon of Western literature. Shakespeare was an exceptionally well known and influential author. His works are quoted and alluded to countless times in several other works of literature, and so part of the importance of this work is how many other people know it and how many other times it is referenced in people's culture, speech, and writing.
Romeo and Juliet has also remained a classic because the central themes and problems are still very relatable, even to contemporary audiences. Audiences can always sympathize with young, impulsive love, and Romeo and Juliet themselves are still very obviously and recognizably teenage. Most teenagers can still sympathize with characters who fall in love with someone their parents don't approve of, and the lengths that the characters go to in order to pursue their love behind their parents' backs.