That is because your Lord would not destroy the cities for wrongdoing while their people were unaware.
The Saheeh International Quran Translation
ذَٰلِكَ أَن لَّمْ يَكُن رَّبُّكَ مُهْلِكَ ٱلْقُرَىٰ بِظُلْمٍ وَأَهْلُهَا غَٰفِلُونَThat is because your Lord would not destroy the cities for wrongdoing while their people were unaware.