Muslims consider the Quran to have been revealed from God to Muhammad. Thus, the language is considered to be a divine message and inimitable. The Quran is also considered the finest literary work in Arabic.
The Quran was reveled in the first half of the 7th century and was originally memorized. Over the next 20 years or so after Muhammad's death, it was collected and written down.
The language is old. The structure of its language is poetical and quite complex. Plus, it was revealed to a non-technological society. It includes words that are no longer used and usage of still extant words that are no longer commonly know. Particularly unusual words are known a gharib or gharibayn, meaning strange, Dictionaries are a great help in bridging the gap between the language of the Quran and the Arabic one learns in classes.