Why did harmony become less able to define musical form during the 19th century?

During the 19th century, the capacity of harmony to define musical form—a role it held firmly during the common practice period—was significantly undermined by several evolving compositional practices.12

The primary factors contributing to this shift included:

• Increased Chromaticism: Composers increasingly utilized chromaticism to enhance emotional expression and color. By incorporating notes outside the basic key, they blurred the listener's perception of a stable tonal center.23
• Ambiguous Chords: The use of complex, unstable chords—such as augmented sixth chords and diminished sevenths—became more frequent. These chords often functioned ambiguously, failing to resolve in expected directions and thereby obscuring the sense of tonality.34
• Evasion of Cadences: Traditional musical form relied on cadences to establish harmonic direction and structural stopping points. Romantic composers, most notably Wagner, frequently evaded these resolutions to sustain dramatic tension, making it difficult to regard key as a unifying force.13
• Expanded Scope and Modulation: As works grew in length and complexity, wide-ranging modulation between remote keys made the classical reliance on a single home key less viable for maintaining structural unity.15