i asked a similar question yesterday, from what i was told and what i already kinda knew is that it was explained in the game that Zant and Ganon are connected to each other, and Ganon in a way gave power to Zant so he can rule the tiwli realm, but what ever Zant wanted/wished Ganon would get also, so at the end of the game, it was Zant who appeared. Zant "whished" to die so his neck broke and since ganon and zant are connected Gannon "died" to.
Zant couldn't really die from the sword of Link, cause if you remember at the end of that battle Zant said, Ganon can bring him back to life.
if my wording was confusing here is what i was told about the topic:
I don't remember why the Realm(s) thingy... but about the neck of Zant, it was because when they met, Ganondorf told him that ANY desire Zant had will be Ganondorf's too.. so, at the end of the game, Ganondorf wasnt really dead with your sword inside (kind-of) so Zant's spirit appear and "WISH" to kill Ganondorf and that's why he broke his neck (in desire, for Ganondorf to do so)