Well, that explains that...

Retired Numbers
Of all the great players that have worn a Red Sox uniform, five of these men have earned the honor of having their numbers retired by Boston. Besides Jackie Robinson, whose number was retired by Major League Baseball in 1997, only Bobby Doerr (1), Joe Cronin (4), Carl Yastrzemski (8), Ted Williams (9), and Carlton Fisk (27) have seen their numbers posted on the right field façade above the right field grandstand. The Red Sox employ a rather strict policy on retiring uniform numbers; to be considered, you need first to have played a minimum of ten years with the team and you must also be elected to the Baseball Hall of Fame. At one point, there was also a requirement in place that a player had to finish his career with the Red Sox, but that rule was dropped after Fisk was elected to the Hall in 2000.
The Sox also use to place the numbers in the order that they were retired;
after Yastrzemski had his number retired in 1989, the numbers went 9-4-1-8Somebody pointed out that the sequence might be interpreted as September 4, 1918 - the day before the first game of the 1918 World Series, which witnessed the last world championship won by Boston before they finally tasted victory again in 2004. This change also occured after number 27 was retired. (Thanks to loyal reader Rex B. from Maryville, TN who pointed out this feature!)
This article was borrowed from Fenwayfenatics.com