Why did violins replace viols as standard ensemble instruments?

The transition from viols to the violin family as the standard for ensemble music was driven by the violin's superior acoustic properties and its suitability for evolving musical styles.12

Several key factors facilitated this shift:

• Tone and Projection: Unlike the reticent, impersonal, and hollow-sounding tone of the viols, the violin offered a "singing" quality, sprightliness, and a more assertive attack. The inclusion of a sound post—a small vertical stick inside the body—gave the violin a lively response and a more powerful, projected sound.12
• Technical Versatility: The violin’s four strings, tuned in fifths, allowed for a uniform fingering technique that reduced the need for complex string-crossing. Furthermore, the absence of frets on the fingerboard allowed for more flexible and expressive playing compared to the fretted viols.12
• Musical Evolution: As composers moved away from older polyphony toward styles emphasizing expressive solo melodies and, later, complete harmonies, the violin proved to be the ideal instrumental counterpart.12

Ultimately, the viols were unable to compete with the violin's expressive capabilities, leading to their decline in favor of the violin-based orchestra.12