Epistolary novels were very common at the time of writing and very popular. A number of novels, including those by Wilkie Collins and by Bram Stoker, were entirely written in letter form, and so we can see that Shelley is imitating this style. However, let us also consider the function of this. It allows us to see how in the framing narrative Walton is profoundly changed by his encounter with Frankenstein. Having shown himself to share a similar obsession to Frankenstein to penetrate the border of man's ignorance around the world, at the end we see him learning a lesson from Victor and returning without having achieved his goal.
I'm not sure myself, though perhaps the answer lies in what it means to communicate by letter? By this I mean that Margaret is passive and cannot elicit responses from Walton in the same way he can from Victor (note how Victor interjects at certain points). A face-to-face encounter with Margaret would have perhaps resulted in some elements being censored by Walton. Subsequently, we have to question Margaret's role in the novel. After all, Frankenstein was intended for her. His letters at the start introduce us to him and, critically, to her as well. She seems to be fairly grounded in personality and being 'Mrs' suggests she has embraced domesticity. Just a few ideas for you.
I need answer about epistolary form in frankenstein
The very outer frame of the novel is epistolary.(in the form of letters) Why did Shelley used this form?