Since WC2 the horse has been iconic to the death knight. In WC3 this was continued with the death knight hero unit including Arthas. Going into WoW more death knights more horses but Arthas just suddenly stopped being seen with his risen steed at least till it’s a raid drop.
I want to say now I get it. 
you do see it when doing the blood elf heritage armor questline
Yeah but that’s just reliving the past.
I think Wrath Arthas just ditched Invincible so he could be a Scooby Doo villain with his Death Gates.
And he would have gotten away with it too, if it wasn’t for you meddling kids.
Lich King powers made him too chunky and he didn’t have gravity powers like Radahn.
I don’t think he ditched Invincible at all, seeing his drops shows it was there all along. 
You just couldn’t see it, because it was invincible.
/OldJoke
You got the joke, you win!