I have been thinking lately about why Emily Bronte set Wuthering Heights in the past: the novel begins about 75 years earlier than Emily's time period and ends about 40 years earlier. In other words, she wrote the novel in the mid 1840s and it spans the years from about 1770 to 1803. Translated to today, it would mean starting a book written now in the year 1935 and bringing it up to 1968.
In contrast, series books tend to be set in the present, whenever that present happened to be when they were composed. In Nancy Drew's case, the setting was meant to be timeless, and I remember that one book, written in 1940, had to have its locale changed from England to Mexico because of the outbreak of World War II. Donna Parkers have more of sense of time because Donna does get older in the books and changes and grows.
Carolyn Haywood's Betsy books are set in the present day of their composition (the 1930s and 40s), but apparently reflect Haywood's memories of her own childhood in the very early years of the 20th century.
Since most series literature is written by older people--people in middle age rather than the teen years or childhood--the books would arguably be conservative, grafting the memories and consciousness of a previous generation on to the present. To me, even Donna Parkers that were written in the early 1960s have more the flavor of the 1940s than the 1960s.
All that being said, no book can be timeless, and it's remarkable how much social history is packed into the pages of our favorite books. This is what gives them their texture and appeal, I believe. Often we remember the details more than the plots, which tend to blur. But what doesn't blur is Nancy Drew ordering cinnamon toast! Or Meg in "A Wrinkle Time" under the covers in her attic bedroom with the wind howling all around the house--and then drinking hot chocolate in the kitchen with Charles Wallace.
What details stick with you from your favorite books?