The only one apt to fully answer that question is the One who revealed the Quran to the prophet Muhammad (ppuh) in the first place. All we can do is presume why via verses in the Quran.
And We did not send any messenger except [speaking] in the language of his people to state clearly for them, and Allah sends astray [thereby] whom He wills and guides whom He wills. And He is the Exalted in Might, the Wise.
Surah Ibrahim (14 – 4)
Every prophet is sent to his people in the language of his people. An Arabic-speaking region gets an Arabic-speaking prophet.
That you may warn a people whose forefathers were not warned, so they are unaware.
Surah Ya-Sin (36 – 6)
The most recent prophet known to be sent in the region of Mecca would be Ismael (son of Abraham), which might be well over 2500 years ago. Therefore, there was a need for a messenger, since the way of Abraham (monotheism) had been lost.
And if We had made it a non-Arabic Qur’an, they would have said, “Why are its verses not explained in detail [in our language]? Is it a foreign [recitation] and an Arab [messenger]?” Say, “It is, for those who believe, a guidance and cure.” And those who do not believe – in their ears is deafness, and it is upon them blindness. Those are being called from a distant place.
And We had already given Moses the Scripture, but it came under disagreement. And if not for a word that preceded from your Lord, it would have been concluded between them. And indeed they are, concerning the Qur’an, in disquieting doubt.
Surah Fussilat (41 – 44-45)
Or lest you say, “If only the Scripture had been revealed to us, we would have been better guided than they.” So there has [now] come to you a clear evidence from your Lord and a guidance and mercy. Then who is more unjust than one who denies the verses of Allah and turns away from them? We will recompense those who turn away from Our verses with the worst of punishment for their having turned away.
Surah Al-Anam (6 – 157)
[This is] the revelation of the Book about which there is no doubt from the Lord of the worlds.
Surah As-Sajda (32 – 3)
Since the Arabs had not received a scripture previously, they had the chance to believe with a book that is in their language, transmitted by a prophet from amongst them. Nowadays, the meaning of the Quran is translated into every major language, accessible on the internet, free of charge. So the same opportunity is offered to the entire world.
Not only was the Quran revealed in Arabic, but in the Arabian Peninsula… What I mean is that it was first presented to a people without history, without civilization, without resource, without significance. They were smack in between the Roman and Persian Empires, and neither could have cared less about them before revelation.
With faith, within a few years, Islam became a beacon of light (justice, peace, and rule) that emanated East and West, only to fade when faith within the individuals faded.
For each one are successive [angels] before and behind him who protect him by the decree of Allah. Indeed, Allah will not change the condition of a people until they change what is in themselves. And when Allah intends for a people ill, there is no repelling it. And there is not for them besides Him any patron.
Surah Ar-Ra’d (13 – 11)
For better, and for worse.