This is because your Lord would not destroy the (populations of) towns for their wrong-doing (i.e. associating others in worship along with Allah) while their people were unaware (so the Messengers were sent).
این (فرستادن رسل) برای این است که خدا اهل دیاری را تا (اتمام حجت نکرده و) آنها غافل و جاهل باشند به ستم هلاک نگرداند.