Because the S prefix guns were made post war to 1966 and are older
Older S&W's are generally more desirable to collectors than newer versions of the same gun.
On a side note there are also S serial K frames in the early post war M&P.
Older is better. All other factors being the same, the older version of a gun will fetch more money. People swoon over the term "S serial" but all that means is that it was made before 1970.
Same with barrels. Shorter is better. A 4" gun will outsell a 6" everyday.