You can add phonetic reasons: aspirated [h] and syllable stress.
historic, historical. The Columbia Guide to Standard American English. 1993
"A historic" or "an historic"?
English Grammar: ENGLISH GRAMMAR: TOP ENGLISH GRAMMAR QUESTIONS - ANSWER PAGE FOR Is it correct to use a and an interchangeably in the following sentence? That is a/an historic part of town. If not, which would be the correct determiner?