Why wouldn't you use Kyurem over Glaceon? Sadly the only things Glaceon really has are it's abilities (which definitely are better than Pressure for Kyurem and both are annoying in Hail), Baton Pass, and Wish. If you're using more of an attacker like Choice Scarf or Specs use Kyurem, otherwise Glaceon's slower and is only equally bulky to offensive variants of Kyurem (since Glaceon is investing max hp) and should there be any investment, Kyurem has much better surviveability. Glaceon really serves no purpose with Kyurem unless you're dualing up on Ice Types...which is not always recommended considering common types (Fire/Water/Steel/Ice etc that resist). Dragon/Ice hits more and has Focus Blast and even amazingly Kyurem has a bigger offensive movepool than Glaceon and can attack both sides (although unless it gets Ice Shard which would have been amazing, the only common physical it gets is Outrage).
Kyurem also is the ultimate lure for things like Scizor and steels you want gone and can crack Blissey unlike Glaceon with Outrage (if you use it). And it sets up Screens that prevent Bullet Punch/Mach Punch from potentially 2 hit koing you, some even with Stealth Rock (Kyurem is a defensive behemoth that hits insanely hard. With Wish support, he can be insanely hard to kill. And thinking about it, Stealth Rock is a meh excuse for it to not be used since Rayquaza/Reshiram/Ho-Oh are weak to Stealth Rock and ARE Uber. And it doesn't really kill Ulgamoth most of the time. Admittedly, they have better movepools/can't be switched in on safely but Kyurem does hit all types but steel and even a lot of steels very hard with Focus Blast or Choice boosted Blizzards which 2 hit ko the lower special defensive steels like Scizor/Metagross as well as Nattorei because of neutrality). With defenses (417/256 Def Reflect, it survives 2 hits from Choice Band Scizor Bullet Punch, even with Stealth Rock, as long as it has Leftovers Recovery going on), Kyurem just seems better.
But Glaceon just looks adorable and some people think Kyurem is ugly (can't imagine why, it's an awesome Ice/Dragon!)
The only things I can imagine that Glaceon can do better than Kyurem are Snow Cloak (could be haxxy with those Blizzards pounding everything into the dirt like Kyurem and turn the thing around) as well as Ice Body although I still think Walrein is likely to pull that set off better because of Steel neutral as well as Super Fang/Brine Combo stalling combo and being really tough to kill either side. Baton Pass could pass either Cheer Up, Curse, or Barrier. But as a straight out attacker, Kyurem is most of the time just better.