ذَٰلِكَ أَن لَّمْ يَكُن رَّبُّكَ مُهْلِكَ ٱلْقُرَىٰ بِظُلْمٍۢ وَأَهْلُهَا غَٰفِلُونَ
That is because your Lord would not destroy the cities for wrongdoing while their people were unaware.
That, sending of the messengers, is because (an, [phonetically] lightened, with the lām [of li-annahu] implied, thus [read as] li-annahu, ‘because’) your Lord would never destroy the towns through injustice, on their part, while their inhabitants were heedless, not having had any Messenger to make [things] clear to them.