Surah 25. Al-Furqan, Ayah 5

Because it was known to his contemporaries that he was unlettered (ummi) and could not read and write.

 Some Arab pagans claimed that the Prophet (ﷺ) had been taught the Bible by some non-Arab Christians. The Quran itself (16:103) refutes this claim by arguing that a non-Arab could not come up with an eloquent Arabic scripture like the Quran when eloquent Arabs failed to produce something like it. For more details, see the Introduction.

In their misguided arrogance they say: 'We have heard such things before; they are pretty tales which have come down from ancient times; they are good for amusement, but who takes them seriously?' When the beauty and power of the Revelation are pointed out, and its miracle as coming from an unlearned man, they again hint at other men who wrote them, though they could not produce any one who could write anything like it.