Alonso is a top 50 MLB prospect while Anderson is more of a top 100, and Rizzo is years away from the majors. Alonso has separated himself from most 1B prospects by having a much better OPB than most, which usually means he's a more finished hitter, and more MLB ready. Every team may have a 1B prospect, but Alonso is easily in the top 5 in the majors among 1B prospects. Plus the Sawks love high OBP guys.
And remember Elsbury is damaged goods both physically and in his relationship with the Sawks. He will not be a Red Sox next year, and everyone knows it. His trade value is much lower than it has ever been.