why do they call susannah beck in the summer i turned pretty

They call Susannah "Beck" in The Summer I Turned Pretty because Beck is her maiden name. Laurel, Susannah's longtime best friend whom she met in college, uses the nickname "Beck" as a term of endearment and a reminder of their deep and long-lasting friendship from before Susannah's marriage. This nickname symbolizes the strength and closeness of their bond, with Laurel being the only character who still calls her by that name in the show. The use of "Beck" highlights the special history and sister-like relationship between Laurel and Susannah.