Why are certain competitions called a "Classic?"

Solution 1:

A classic is an outstanding example of a particular style, something of lasting worth or with a timeless quality. … Classic is used to describe many major, long-standing sporting events.

(src: WP, see link below).

See also:

• The section on Sport in the Wikipedia article 'Classic:'

Many sporting events take the name classic …

• AHD cited in TFD

6. A traditional event, especially a major sporting event that is held annually: a golf classic.

Solution 2:

The noun classic means something that's very high quality, particularly if it has lasting value. (vocabulary.com)

• The pristine Model T Ford that you keep in your garage is a classic

Classical is the customary word when reference is made to the arts and literature of ancient Greece and Rome (a classical scholar, classical Latin, classical metres).

• The works studied, and also the subject itself and the study ofthe subject, are called (the) Classics. (Fowler's Modern English usage)

Classical is also applied to serious or conventional music (i.e. that of Beethoven, Mozart, etc.) as distinct from light or popular music; and in physics to the concepts which preceded relativity and quantum theory. (Fowler's....)

classic (adj.) 1610s, "of the highest class; approved as a model," Originally in English, "of the first class;" meaning "belonging to standard authors of Greek and Roman antiquity" is attested from 1620s. (etymology.com)

• Classic means 'of acknowledged excellence' (the classic textbook on the subject), or 'remarkably typical' (a classic case of cerebral palsy)