Amina Inloes, Amina Inloes is originally from the US and has a PhD in Islamic Studies from the University of Exeter on Shi'a hadith. She is the program leader for the MA Islamic Studies program at the... Answered 1 year ago

Some people say it is because Allah had created the jinn first, and the angels had seen the jinn fighting so knew that a new species with free will would also fight.

Some say that there were other humanoids (nasnas), and they had fought, and so they deduced from this that a new species with free will would also fight.

These two views are reinforced by a narration from Imam al-Sadiq (A) in which he says that the angels could not have known that humans would cause bloodshed if they hadn't seen the precedent of earlier creatures on earth.

Some simply say that they understood this from looking at human nature, or by looking at the nature of life on earth and the challenges of living as an earth-based being (for instance, competition for food, water, reproduction, and land).

Since the earth (dunya) is considered the basest or lowest level of creation, vis-a-vis the heavenly realms, perhaps it also didn't make sense to instill Allah's vicegerency in an earthling. 

Maybe there are other reasons too which relate to the nature of the angels and their existence outside of the earthly realm which would give them access to knowledge or a perspective that human beings generally lack.