The Quran

Preface

وَما مَنَعَنا أَن نُرسِلَ بِالآياتِ إِلّا أَن كَذَّبَ بِهَا الأَوَّلونَ ۚ وَآتَينا ثَمودَ النّاقَةَ مُبصِرَةً فَظَلَموا بِها ۚ وَما نُرسِلُ بِالآياتِ إِلّا تَخويفًا

Yusuf Ali

And We refrain from sending the signs, only because the men of former generations treated them as false:¹ We sent the She-camel² to the Thamūd to open their eyes, but they treated her wrongfully: We only send the Signs by way of terror (and warning from evil).³

Past generations treated Signs and Portents with contempt or rebellion, and brought about their own undoing. It is only God’s Mercy that gives them Grace for a time and prevents the coming of those Portents and Punishments which would overwhelm them if they were put to their trial at once.
An example is cited from the story of Thamūd. A wonderful she-camel was sent among them as a Portent and a Symbol. In their wickedness they hamstrung her. So instead of her reclaiming them she was a cause of their destruction, as their sin and rebellion were laid bare. For the story of the she-camel and the reference to the passages in which she is mentioned, see n. 1044 to 7:73.
Signs, Miracles, and Portents are sent by God as a warning, to strike terror into the hearts of evildoers and reclaim them to the right path. I have discussed Fear as a motive for reclaiming certain kinds of hard hearts, in my note 82 to 2:74. But some hearts are so hard that even this motive does not work. As they have a limited free will given by God, they are to that extent free to choose. But when they actually choose evil, God in His infinite Mercy delays their punishment and removes the occasion for their immediate self-destruction by withholding the Signs which might make them transgress all the more and compass their total destruction.