Answer:Because they don't have five feet in the line or don't have an unstressed syllable followed by a stressed one like iambic pentameter requires.
Explanation: In poetry, an Iambic Pentameter is a metrical speech that has 5 feet on every line.A foot is a pair of syllables in which there is an unstressed syllable followed by a stressed one.
The lines "As wicked dew as e'er my mother brushed"and "With raven’s feather from unwholesome fen" follow the rules of iambic pentameter, while the line "Upon thy wicked dam, come forth!" has just four feet, and the line "Drop on you both! A southwest blow on ye" doesn't have the unstressed syllables at first.
