Why did Mary Shelley choose a frame narration for Frankenstein?

Shelley's framing of Frankenstein allows for this deep exploration of themes because these frames put the similarities between the characters in conversation and suggests the threat of the narrator of the fluid, directing frame giving into the influences or vices of the more temporally fixed, interior frames.

Explanation:

what are the phrases to choose from?

martians or other types of aliens represent a unique fear as opposed to zombies. vampires and other types of monsters.   in most sci-fi stories, aliens come from other worlds and have different or more advanced technology.   abductions and experimentation on human bodies that involve these technologies, or the use of these technologies against human beings and the use of unknown "powers", are a type of fear that's different from that of other types of monsters.