It's funny, but yes, why didn't she simply use this ridiculously powerful spell, to you know, get the artifact? It's one of those moments that breaks the plot, but eh, it's a joke ending.
Well the spell does have limits, usually at a D&D table those limits are left to the DM to determine because the spell description doesn't really draw too many specifics. But stuff like messing with the gods would probably not work. The artifact is also an item of immense power sought by some gods as well, like Shar. And the game also defines the artifact as sentient. Once the artifact goes to your rescue if Shadowheart isn't in your party it stays with you and asking Shadowheeart if she wants it back results in her saying it may not be possible because the artifact is kind of stubborn and does what it wants. So it's entirely possible that getting this artifact may well be beyond the abilities of a wish spell...that's not really a narrative problem as far as I can tell.
