Why was Ganondorf sealed and not killed?

I look at it this way. With exception of Rauru, nobody knew anything of the sacred realm when Ganondorf first entered. When the Sacred Realm was changed into the dark world, apparently there were more lifeforms than just Rauru. The fairy there gave the silver arrows to Link. You would figure that Rauru should have known about the silver arrows before Link in ALTTP received them from the fairy.

Or you could figure that they knew nothing on how the Triforce could be used for evil, so they figure light will smite the dark, thus creating the light arrows.

Every time the light arrows are used, they just cause Ganon(dorf) pain, never kill. After so long, the realization was made that the silver arrows can kill Ganon. Because he had all that time in the dark world to gain power from the complete Triforce, it took 4 arrows to do the job.

Possibly because he only had the ToP in LoZ, it would only take one to finish him off.