Take the C-elements for example: C, Ca, Cd, Ce, Cf, Cl, Cm, Cn, Co, Cr, Cs, Cu.
Imagine some newly synthesized element is to be named after the University of Colorado: Coloradium. Its symbol can't be C, Co, Cl, Cr, Ca, Cd, Cu, or Cm, so we're forced to choose Ci.
Then another new element is synthesized, and it is to be named after Celine Dion: Celinium. Its symbol can't be C, Ce, Cl, Ci, Cn, Cu, or Cm. What would they choose for a symbol?