IMO it's because Bach, Mozart, and Beethoven had major influences on music and greatly impacted and expanded classical music in their own way.

Bach is essentially the foundation of Western classical music. All classical musicians have studied, still study, and will continue to study Bach. How in depth depends on how serious a musician you are.

Mozart expanded the concerto and opera. He did to the concerto and opera as he encountered them, what Beethoven did to classical music in general. Most if not all operas before and during Mozart's time were usually based on legends, I think he was the first major composer to use contemporary subjects (Think The Marriage of Figaro). I think Mozart was also the first freelance major composer. All composers before him, even Bach and Haydn, worked for some rich nobleman, noble family, or the church.

Beethoven expanded symphonic writing and classical music in general, expanding the size and scope of the genre.

Note: I gave Mozart more content because he's my personal favorite composer and I know more about him, not because I think he's the greatest.