Qur'an was revealed in 7th century Arabia in the region of Makkah. It was simply revealed in the language of region of that people of that time. As clearly stated in 44:58
فَإِنَّمَا يَسَّرْنَاهُ بِلِسَانِكَ لَعَلَّهُمْ يَتَذَكَّرُونَ
Thus We have revealed this [Qur'an] to you in your own tongue so that they may take heed.
It was a coincidence that it was Arabic, had it been English or French or Urdu it would have revealed in that language. But now, since the revelation was done in a particular language, care had to be taken to preserve the dialect, idioms, sayings, poetry of that language so that the correct understanding of Qur'an can be reached. This was always a priority for our scholars. As in the following Narration of Ibn Abbas (Suyuti, al-Itqan fi 'Ulum al-Qur'an, 1st ed., vol. 1 (Beirut: Dar al-Fikr, 1996), 347)
إذا سألتم عن غريب القرآن فالتمسوه في الشعر فإن الشعر ديوان العرب
If you want to understand the meaning of a Qur'anic word little known to you, search for it in poetry because it is this poetry which is the anthology of the Arabs.