Why is emigrate spelled with one m while immigrate has two m's?

immigrant (n.) Look up immigrant at Dictionary.com
"one who immigrates," 1792, American English, perhaps based on French immigrant, from Latin immigrantem (nominative immigrans), present participle of immigrare "to remove, go into, move in" (see immigrate). Emigrant is older. First used in English in Jeremy Belknap's history of New Hampshire, and he generally is credited with having coined it.

There is another deviation from the strict letter of the English dictionaries; which is found extremely convenient in our discourses on population. From the verb migro are derived emigrate and IMMIGRATE; with the same propriety as from mergo are derived emerge and IMMERGE. Accordingly the verb IMMIGRATE and the nouns IMMIGRANT and IMMIGRATION are used without scruple in some parts of this volume. [Preface to vol. III of "The History of New Hampshire," Belknap, 1792]

As an adjective from 1805.

SOURCE: www.etymonline.com/index.php?term=immigrant

In all cases the prefix "e" means out and the prefix "in" or "im" means in.