Arabic was the most advanced and comprehensive language at the time when Quran was being revealed and it still is the most comprehensive. The terms, concepts and themes would be best clarified and explained in Arabic language and last but not the least, it was the language of the people where it was revealed. Allah said in the Quran:
1. Arabic was the language of the people where it was revealed & also the language of prophet Muhammed (p.b.u.h)
وَكَذَلِكَ أَوْحَيْنَا إِلَيْكَ قُرْآنًا عَرَبِيًّا لِتُنْذِرَ أُمَّ الْقُرَى وَمَنْ حَوْلَهَا وَتُنْذِرَ يَوْمَ الْجَمْعِ لا رَيْبَ فِيهِ فَرِيقٌ فِي الْجَنَّةِ وَفَرِيقٌ فِي السَّعِيرِ
“And thus: We have revealed to you a Quran in Arabic so that you may warn the Foremost of all towns and those who dwell around it, and may warn of the Day of Gathering, which is beyond all doubt. One group will be in the Garden, and one group will be in the Flames.” (Quran 42:7)
2. To explain it without any want of clarity, distinctness or perspicuousness
وَلَوْ جَعَلْنَاهُ قُرْآنًا أَعْجَمِيًّا لَقَالُوا لَوْلا فُصِّلَتْ آيَاتُهُ أَأَعْجَمِيٌّ وَعَرَبِيٌّ قُلْ هُوَ لِلَّذِينَ آمَنُوا هُدًى وَشِفَاءٌ وَالَّذِينَ لا يُؤْمِنُونَ فِي آذَانِهِمْ وَقْرٌ وَهُوَ عَلَيْهِمْ عَمًى أُولَئِكَ يُنَادَوْنَ مِنْ مَكَانٍ بَعِيدٍ
“Now if We had made it a Quran in a non-Arabic tongue they would surely have said, “Why is it that its verses have not been made clear? Why – a foreign tongue and an Arab?” Say, “For those who accept it, this is a Guidance and medicine for a wholesome life. But as for those who will not believe (Arabs or non-Arabs), in their ears is deafness, and so it remains obscure to them. They are like people who are called to from afar.” (Quran 41:44)
3. To make it easy to understand
إِنَّا أَنْزَلْنَاهُ قُرْآنًا عَرَبِيًّا لَعَلَّكُمْ تَعْقِلُونَ
“Behold, We have sent it down in all clarity, in the Arabic tongue, so that you might encompass it with your reason.” (Quran 12:2)
إِنَّا جَعَلْنَاهُ قُرْآنًا عَرَبِيًّا لَعَلَّكُمْ تَعْقِلُون
“Behold, We have made it a Quran in clear Arabic language that you may fully understand.” (Quran 43:3)
It should be noted that there are around 80,000 words in the Quran but the actual words are only around 2000. In principle, if you take out the repeated words, the total number of words in Quran may well be less than 2000. Allah has revealed His book which contains so few words. Accordingly, if a reader decides to learn only 10 new words everyday, he can understand the basic message of the Quran within a period of seven months! So it is indeed very easy to understand the Quran, provided one is willing to learn it.
The challenge of the Quran, is to produce a single Surah (chapter) like the Surahs it contains. The challenge to produce a Surah, which, in beauty, eloquence, depth and meaning is at least somewhat similar to a Quranic Surah remains unmet to this day. Any scripture claiming to be a divine revelation must also be acceptable on the strength of its own reason and logic. The Quran is not a book of science but a book of ‘signs’, i.e. ayats. There are more than six thousand ‘signs’ in the Quran of which more than a thousand deal with science.
https://islamgreatreligion.wordpress.com/2011/09/09/why-quran-was-written-in-arabic-language/