How did chromaticism weaken the role of key contrast in late-19th-century sonata form?

In late-19th-century music, the role of key contrast in sonata form was weakened by the extensive use of chromaticism—the inclusion of notes and chords foreign to the established key of a composition.1

During the Classical era, sonata form relied on the clear definition of a tonic key and the dramatic tension created by modulating to and from contrasting key areas. However, as composers like Richard Wagner expanded their harmonic vocabulary, the frequent use of chromatic tones and chords obscured the listener's sense of tonality. When chromaticism becomes pervasive, the music no longer functions as a clear departure from or modification of a home key; instead, the key itself fails to establish enough stability to serve as a structural anchor.2314

Consequently, the traditional reliance on the contrast, conflict, and eventual reconciliation of keys as the primary organizing principle of sonata form diminished. Many later works shifted their focus toward melodic development, thematic transformation, or programmatic narratives rather than the structural interplay of keys that defined the Classical tradition.13