In regards to The Scarlet Letter, why does Hawthorne not tell us what the scarlet letter means as he does with many of his other color symbols?

It is in his use of symbols in The Scarlet Letter that Hawthorne has made one of the most significant contributions to American literature.  Certainly, his novel is regarded by many as the first symbolic novel to have been written in the United States.  In fact, it is Hawthorne's use of the symbols of the scarlet A and Pearl that have come to define symbolism as we know it:  A symbol means what it is and more; it functions both literally and figuratively at the same time.  Thus, the symbol is the richest and at the same time the most difficult of the literary devices.  For both its richness and its difficulties result from its imprecision. It is this very imprecision which Hawthorne has created with his two major symbols.  With this imprecision, his scarlet A defines an area of meaning and interpretation that falls within that allowed area.

The scarlet A takes on various appearances and meanings as Hawthorne's narrative develops.  Of course, the first meaning of the A is that of adultress as Hester stands on the scaffold in the beginning chapters.  However, the letter appears in many forms and places.  For instance, it is elaborately embroidered in gold, a weight upon Hester's heart on which Pearl throws wild flowers in Chapter VI.  Magnified in the armor breastplate at the mansion of Governor Bellingham in Chapter VII, it is viewed

in exaggerated and gigantic proportions, so as to be greatly the most prominent feature of her appearance.

Truly, the scarlet letter has become a point of shame and ostracism for Hester. Later, in Chapter X, Pearl again decorates Hester's letter by making a border of "prickly burrs from a tall burdock which grew beside the tomb."  The letter now tortures Hester.  On the night of his vigil on the scaffold in Chapter XII, Dimmesdale sees the immense red A in the sky and feels his shame, while others see it and interpret it as meaning "angel" on the night of Governor Winthrop's death.  Of course, the most dramatic significance of the A is the letter that lies in the flesh of Dimmesdale's chest in Chapter XXIII.  Finally as a summary symbol, there is the allusion to the scarlet A against the black background on the tombstone of Hester and Dimmesdale in Chapter XXIV.

So, not only does the A appear in various forms, but it has various meanings.  From the original mark of adultery, it is a mark of just punishment to the grey Puritans, but one of unjust humiliation to Hester; to Dimmesdale it is a cruel reminder of his own sin; to Pearl a mystery and curiosity.  However, the letter even has an extension of meaning to "Angel" and "Able" when Hester, at last, earns some respect from the Puritans.  Yet, upon her return to American, she bends and puts it back on her breast, making the scarlet A the quintessential symbol.