The 'h' is stressed in 'history', not in 'historic'. Try pronouncing 'historic' with a stress on the first syllable, it sounds wrong. Since it's softer in historic it's more natural (to me anyway) to use 'an'.
(Also, I'm french so I barely pronounce h's to begin with - so in my speech the stressed 'h' in history is kinda soft, and the unstressed h in historic is barely there.)
History: /ˈhist(ə)rē/
Historic: /hiˈstôrik/
The 'h' is stressed in 'history', not in 'historic'. Try pronouncing 'historic' with a stress on the first syllable, it sounds wrong. Since it's softer in historic it's more natural (to me anyway) to use 'an'.
(Also, I'm french so I barely pronounce h's to begin with - so in my speech the stressed 'h' in history is kinda soft, and the unstressed h in historic is barely there.)
(edited)