Retiring a number takes something out of use permanently so that nobody else can wear it. The custom is American in origin and unusual among the ways sport commemorates people.
Numbers had to matter first
Shirt numbering was introduced to help spectators and officials identify players, and it was resisted in several sports as an indignity before becoming standard.
Once numbers were fixed to individuals rather than to positions, they became identifiers, and supporters associated a number with a person.
That association is the precondition for retirement. A number that merely denotes a position cannot commemorate anybody.
Withdrawal is a stronger gesture than display
Most commemoration adds something, such as a statue, a plaque or a name on a wall, which can be added indefinitely without cost to anyone.
Retiring a number instead subtracts, permanently reducing what the club has available and imposing a small ongoing cost on every future squad.
That cost is the point. A gesture that constrains the institution forever carries more weight than one that occupies wall space.
Scarcity forces clubs to ration it
A team has a limited supply of numbers, and clubs with long histories have retired enough of them to create genuine practical difficulty.
Some have set criteria, restricted the practice to exceptional cases or introduced honors that fall short of full retirement.
Others use the number for a season and then withdraw it, or allow a family member to wear it, which are compromises the original custom did not anticipate.
The custom traveled unevenly
Sports where numbers correspond to positions found the practice difficult, since removing a number from circulation interferes with how a team is organized.
Soccer clubs adopted it selectively after squad numbering became permanent, which is what made a number personal rather than positional.
Sports without fixed numbering commemorate differently, through named trophies, honors boards or awards, arriving at the same purpose by other means.
It creates an argument about who qualifies
Because the gesture is permanent, deciding who receives it becomes contentious, and clubs face pressure from supporters, families and former players.
Decisions made under emotion shortly after a career ends look different decades later, and reversing one is nearly impossible.
The permanence that gives the custom its force is also what makes every instance of it a judgment the institution has to live with.

