Will give you the reasons from different perspective:

Language: Quran language is Arabic and Allah “God” challenged all Arabs to give one verse like Quran and they couldn’t despite the language skills they had and historically proofed by poetry.

Muhammad the messenger can’t read or write.

Give me 100,000 who memorizes Bible or Torah, and I’ll give you one billion people memorizes Quran.

Sociolay, few examples, divorce allowed in Islam from day one, while was forbidden in Christianity, and now allowed in most Churches why?

Interest forbidden in Islam and economically they said interest is the reason for all economic disasters.

Liquor is forbidden in Islam, and you can see how societies suffer from alcoholics.

Scientifically, black-holes mentioned in Quran.

God said in Quran we made from water every living thing, and now they realized if they need to find life they have to proof water is exist.

Knocking star mentioned in Quran.

Spherical earth mentioned in Quran.

The above are examples of Quran miracle, how did Muhammad the messenger knew all this while he is not reading or writing, no doubts high power Allah give him Quran.