So, with all the new Shadow Isles stories we now know that such a dreadful and grim place wasn't always like that.
Before balance was broken by the ruined king's sorcery, they were the Blessed Isles.
So... how come Bard didn't intervene? 
(Non-red-name speculation follows - I do not have an official answer!)
The Ruined King is mentioned in the Bard "The Wonder Above" teaser (the one with the constellations), although not directly suggesting any involvement, each constellation speaks of an unnatural state that was then prevented or ended.
http://na.leagueoflegends.com/en/champion-preview/wonder-above
Maybe Bard was there, and through his actions the resurrection of the Ruined King's wife went as badly as it did. Maybe the options were:
Maybe an undead army is the lesser evil, compared to being able to resurrect anyone? If the undead piss off Kindred, imagine how much true resurrection would!
(Also, bonus - the entry mentions that mortality should be pondered via poetry and verse. I'm happy that Lamb is a poetry lover :D)
He might have been unable/unaware of that. Bard isn't almighty,he doesn't see everything(for example he only noticed where the egg was once it was used) and can't be everywhere at once.
He most likely did in some random way, but he doesn't just walk up, backhand someone and say "No." He probably stole away one or two specific items.
Why would he? Bard only comes in to maintain a "cosmic balance" (whatever that means). As far as we know, there hasn't been any colossal and cosmic imbalances since the Shadow Isles had been formed. In fact, I think the Shadow Isles creation was a force that is truly negative for the first time since 
was first raised through Necromancy. It serves as a great balance and contrast to a land like Ionia which has been seemingly blessed with wisdom and knowledge in the pursuit of things that are deemed good.
Because Bard cares about the entire universe, not a single island.
Well how about this, what if he chose not to.
As in he let it happen. I have no real evidence to say this is true, as well this is a far out theory if anything, but Bard works on a different level that we are aware of. He knows whats suppose to happen so who's not to say for a future event to happen he needs the Harrowing to exist or endanger the future balance.
Because he's an asshole.
