The infidels turn away from the signs of Allah (statements of the Quran, natural phenomena, miracles etc.) and deny the truth. Generations before them were destroyed due to their infidelity, they can travel in the earth and witness these historic facts. They are so staunch in their infidelity that if the Quran were written down on pages which they could feel with their hands and sent down even then they would have rejected it. Angels are sometimes sent for punishment 3:125, therefore if an angel is sent, they would be destroyed, besides, for them to see, the angel would have been sent down in the form of a man which would further confuse them. It has been the procedure of the infidels to mock at messengers and ultimately be punished.