Question

Why do authors of magical realism often use a serious tone in love stories

Answer 1

Magic realism is all about creating a setting in which the characters experience magical elements as something common to their everyday lives and not as something fantastic, just like the snow is common from somebody living in the northern hemisphere, but is somehow weird for someone who has never seen it. The serious tone is fundamental for the credibility of the magical elements as part of the everyday life and not as something outstanding. 

Answer 2

Well if you are not looking for an overly complicated answer for Apex, the answer would be "it makes the magical details sound more real and ordinary"