A lot has to do with facing familiar pitchers, outfielders playing in different parks, catchers having to learn a new staff and hitters.
History shows those players don't work out well for the Sox at least a decade. Vegas uses history in determining the odds. Sox scouts think everything works in a vacuum. I guess they can continue down that road that has failed for them much like trying to develop their own catchers. Stats sheets and reality are often opposite for this team. Fans want to believe some veteran guy from the NL is somehow gonna work out but most often they don't. The results are rarely immediate for the Sox.