How can a sonata-form movement have only one main theme?

In sonata form, the number of themes is of minor structural significance compared to the movement's tonal design. While it is common for a movement to feature two or more contrasting themes, a sonata-form movement can certainly function with only one main theme.1

The defining characteristic of a sonata-form exposition is not a specific number of themes, but rather a dynamic contrast of keys, or tonal contrast. The exposition typically begins in the tonic (home) key and moves to a closely related but distinct key, such as the dominant or the relative major. This opposition of tonalities creates the structural tension that the subsequent development and recapitulation sections strive to reconcile.1

Because the form is driven by this tonal journey rather than a fixed thematic count, composers like Haydn have successfully utilized a single main theme in works such as his Symphony No. 85 in B-flat Major. In such cases, the structural integrity of the form remains intact through the clear establishment and eventual reconciliation of contrasting key areas.1