Why are spondees and pyrrhics usually treated as substitutions in English verse?

In English accentual-syllabic verse, the metre is defined by a consistent pattern of stressed and unstressed syllables, typically organized into iambic, trochaic, anapestic, or dactylic feet.12

Spondees (two stressed syllables) and pyrrhics (two unstressed syllables) are treated as substitutions rather than foundational metrical patterns because they cannot sustain the rhythmic structure required for English verse. A spondee does not form a basis for English metre because the language lacks words with truly equal stress, and a pyrrhic foot lacks the necessary stress to maintain the expected rhythmic pulse of a line. Consequently, when these feet appear, they function as temporary variations that replace the prevailing foot of the series, allowing poets to introduce rhythmic nuance without disrupting the overall metrical framework.3145