The Qur’an was revealed in Arabic as that was the tongue of the Prophet صلى الله عليه وسلم and his people. Thus pronouncing the Arabic letters when reciting the Qur’an was natural for them.
Later when Islam spread to non Arab lands, and with it the Arabic-speaking Muslims, the original Arabic they spoke began to become polluted and mixed with the languages and dialects of these places. Even within the Arabian peninsula itself, Arabic ‘changed’ and ‘developed’ as is natural for any language over time and more quickly so when there is contact with other cultures and civilisations.
However, unlike other languages the original pure Arabic spoken at the time of the Prophet صلى الله عليه وسلم was also preserved, due to and via the preservation of the Qur’an.
As a result of this, a form of Arabic remained intact till today that is now known as Modern Standard Arabic (MSA), and which is an official form of the language used and understood by Arabs no matter how much their personal dialects may differ.
And as for the Qur’an, no matter how different their Arabic dialects may have become from the original Arabic, the Reciters would recite the Qur’an in its original pure way. As passed down via the mass oral chains of transmission.
Today, the Arabic dialects spoken across the world are so diverse that it is almost impossible for people from one place to understand the people from another, even though they may all be speaking a form of ‘Arabic’. Yet they all understand Modern Standard Arabic.
The pronunciation of many letters has changed too. For example in some parts of Egypt they pronounce the letter ج like the English hard ‘g’, or the letter ق like the letter ء. Other countries pronounce the letter ق like the letter غ and yet others pronounce it like a hard ‘g’. The letter ظ is pronounced similar to a ز in some places, and the letter ض pronounced as a fat د in some places, or like a fat ل in others, or as a pure ظ, or mixed with it in yet others. Some letters are no longer pronounced the original way anywhere amongst the modern Arabic dialects such that they don’t even pronounce them properly when speaking Modern Standard Arabic.
The original pure pronunciation of all the Arabic letters together, has only been preserved by the Reciters of the Qur’an who are part of the mass oral chains of transmission linked back to the Prophet صلى الله عليه وسلم. And today these certain letters are only heard amongst them when they recite.
Indeed, it is We who sent down the Qur’an and indeed, We will be its guardian
[Surat Al-Hijr, verse 9]