بسم الله الرحمن الرحيم
There are many reasons , but I will only mention one that is the most important and may concern us .
The Arab nation at that time in the Arabian peninsula was made up of many tribes قبائل , and even some of these tribes consisted of smaller ones called butoon بطون ( correct translation needed ! ) . Prophet Muhammad , peace be upon him , was not only sent to all these tribes & sub-tribes ( I coined this word ) but to the whole world which , of course , has other nations beside the Arab . Furturmore , there are sounds in Arabic that cannot be pronounced in the other foreign languages ; that is , they have no equivalents .
To get the extreme variation in dialects within the Arab nation itself , it may be helplful to know that Egyptians for instance cannot easily understand Khaliji ( from the Arabian peninsula ) dialects , nor the Moroccan . Even within the Arabian peninsula the dialects notably vary from one country to another . Even in Egypt , there is the Caireen , the Alexandrian , the Asuti & many other dialects .
It comes , then , as a valid reason to have various Qiraats to correspond to the varieties of the Arab nation and the difference of the world languges and tongues .
We can trace this reason in the Hadithes
" هون على أمتي "
Make it easy for my Ummah (nation)
" أسأل الله معافاته و مغفرته ، إن أمتي لا تطيق ذلك "
... My nation will not bear up with that .
" يا جبريل ، إني أرسلت إلى أمة أمية فيهم الرجل و المرأة و الغلام و الجارية و الشيخ الفاني الذي لم يقرأ كتابا قط "
Gabriel ! I was sent to an illiterate nation ( Ummi ) , amongst which there are men , women , boys , girls & even the aging sheik who never read a book at all
It may be helpful to mark out that although there are Caireen , Alexandrian ,... etc. dialects , they all belong to the SAME broader Egyptian dialect going under Arabic language . It is easy however to distinguish these dialects depite belonging to the same category . This shows how varieties can matter even within the same country . So how about different countries in the same region , or different regions in the world ??
The smae goes for Qiraats which are piled under the Arabic language . There are variations in recitation , uttering sounds , tone , intonation but are all Arabic . Just like varieties within all other languages .