The Quraysh claimed They can produce Something similar to the Qur'an

Allah describes the disbelief, transgression, rebellion, as well as misguided statements that the pagans of Quraysh used to utter when they heard Allah's Ayat being recited to them,

قَدْ سَمِعْنَا لَوْ نَشَآءُ لَقُلْنَا مِثْلَ هَـذَآ

The answer to this was given by the Qur'an itself. First, it was said: وَمَا كَانَ اللَّـهُ لِيُعَذِّبَهُمْ وَأَنتَ فِيهِمْ (And Allah is not to send punishment upon them while you are in their midst [ in Makkah ] - 33). The reason is that it is the customary practice of Allah Ta` ala with all blessed prophets that He would not send His punishment over a habitation where they are present until He takes out his prophets from there. This is as it happened in the case of Sayyidna Hud, Sayyidna Salih and Sayyidna Lut, may peace be upon them all. Punishment did not come as long as they remained in their towns. Punishment came when they were taken out from there. As for the Prophet of Islam ﷺ ، he was sent to this world as universal mercy. That Divine punishment would come on him while he was present in a town was an eventuality counter to his station.

In a nutshell, the answer is: Because of your hostility towards Qur'an and Islam, you very much deserve to have stones rain at you but the presence of the Holy Prophet ﷺ in Makkah prohibits it. According to Imam Ibn Jarir, this part of the verse was revealed at a time when he was present in Makkah. Then, came Hijrah to Madinah where the second part was revealed as follows: وَمَا كَانَ اللَّـهُ مُعَذِّبَهُمْ وَهُمْ يَسْتَغْفِرُ‌ونَ (nor would Allah send punishment on them while they are seeking forgiveness - 33). It means that following his migration to Madinah, though the preventive factor of a mass punishment stood removed as he was not present there in Makkah, yet there did remain another factor preventing the coming of mass punishment even at that time since many weaker ones among Muslims who were unable to migrate had remained behind in Makkah and they were the ones who kept praying to Allah Ta` ala for their forgiveness. It was for their sake that punishment was not sent upon the people of Makkah.