Why is "fulfil" spelt as "fulfill" in American English?

Interesting question :) Reading up on this has been a pretty crazy experience. My conclusions are the following:

• The reason for the change from fulfil to fulfill is attributed to inflection, with the spelling modified to suit the function of the word.
• The reason why it isn't spelt fullfill is similar to why parallel only retains the single 'l' at the end—to curb the apparent fugliness of an '-llell' cluster in words such as unparalleled etc.
• There is also a question of consistency involved.

But, unfortunately, another rule in the AE transition was to preserve the "base" word as much as possible, which is why skill/_skilful in BE became skill/skillful in AE. However, this introduced a cluster of letters in words such as skillfully. To put it mildly, the result of exercise was and remains consistently inconsistent.

A related question on the difference in the AE/BE spellings of propelling covers similar territory. It also points to a WP page detailing the differences between American and British spelling which includes the following section on the madness of doubled consonants:

• The word parallel keeps a single -l- in British English, as in American English (paralleling, unparalleled), to avoid the unappealing cluster -llell-.
• Words with two vowels before a final l are also spelled with -ll- in British English before a suffix when the first vowel either acts as a consonant (equalling and initialled; in the United States, equaling or initialed), or belongs to a separate syllable (British fu•el•ling and di•alled; American fu•el•ing and di•aled).
  • British woollen is a further exception due to the double vowel (American: woolen). Also, wooly is accepted in American English, though woolly prevails in both systems.

There's also a note on -ful and -fil:

In both American and British usages, words normally spelled -ll usually drop the second l when used as prefixes or suffixes, for example full→useful, handful; all→almighty, altogether; well→welfare, welcome; chill→chilblain. The British fulfil and American fulfill are never fullfill or fullfil. Johnson wavered on this issue. His dictionary of 1755 lemmatises distil and instill, downhil and uphill.