Why can stress patterns be measured more precisely than vowel quantity in English poetry?

In English poetry, stress patterns are more easily measured than vowel quantity because English is a strongly accented language where stress—the intensity or relative loudness given to a syllable—serves as a fundamental structural element.12

While quantitative verse (common in classical Greek and Latin) relies on measuring the duration of time required to pronounce syllables, English prosody is primarily accentual-syllabic. In this system, the rhythmic structure is defined by the regular recurrence of stressed and unstressed syllables, which can be clearly identified and translated into visual symbols through scansion.23

Conversely, vowel quantity—the length of time a vowel sound is held—is not the primary organizing principle of English metre. Although poets may use assonance or vary vowel sounds for effect, these features are often secondary to the dominant, measurable patterns of stress that define the foot and the metre of the line.32