﴿ ذَٰلِكَ أَن لَّمْ يَكُن رَّبُّكَ مُهْلِكَ ٱلْقُرَىٰ بِظُلْمٍۢ وَأَهْلُهَا غَٰفِلُونَ ﴾

“And so it is that thy Sustainer would never destroy a community for its wrongdoing so long as its people are still unaware [of the meaning of right and wrong]:”

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.