Why is Janna the only one who can preserve a turret?

Presently, Janna is the only Champion in the game capable of shielding a turret from damage, and oftentimes that doesn't work well enough to stop the turret from being destroyed (not that it should if it's being pushed). The only other way to protect a turret? Kill stuff before it reaches it. There's no other way to actually block the damage the turret takes, leading to a lot of scenarios where you try to defend a turret and die because it collapses.

Seeing as there are presently too many anti-turret mechanics now available in S5, I think there should also be at least one item available that lets you shield a turret from damage for a short time. This would let you kill a minion wave and make your opponents have to do a double take on whether or not they can snipe your turret before you kill them.

Could be helpful as a Support or Tank pickup, or perhaps as a secondary use for Ohmwrecker.