Why does the recapitulation usually return to the tonic key?

In sonata form, the return to the tonic key during the recapitulation is essential for achieving a "reconciliation of opposites" that resolves the conflicts established earlier in the movement.1

While the exposition introduces themes in contrasting keys—typically the tonic for the first subject and a dominant key for the second—the development section creates a state of tonal flux and thematic tension. By restating the subjects in the tonic key, the recapitulation brings the musical material into a new state of equilibrium.12

This structure reflects the historical evolution of sonata form from binary form, where the second part was tasked with moving from a new key back to the original tonic. Ultimately, the return to the tonic provides a sense of finality and synthesis, allowing the listener to perceive the previously conflicting subjects as a unified whole.31