Why was compass declination not widely adopted to find longitude at sea?

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Edmond Halley Edmond Halley, detail of an oil painting by Richard Phillips, c. 1720; in the National Portrait Gallery, London.

The use of compass declination to determine longitude at sea was never widely adopted for two primary reasons:

Measurement Difficulty: It proved extremely difficult for mariners to determine the compass declination with the level of precision required for accurate navigation.1
Secular Variation: It was discovered that compass declination is not constant; it varies over time, meaning the magnetic field changes from year to year.21

While Edmond Halley published magnetic charts of the Atlantic and Pacific oceans in 1701 to assist with this method, these inherent limitations made it an unreliable tool for finding longitude. Ultimately, the development of the marine chronometer by John Harrison provided a more dependable solution for mariners.13