On corp's release everyone and their dog wanted spirit shields. Their stats weren't significantly better than alternative options, except for maybe divine. At the time their special effect was also way shittier than it is now, but despite numerous buffs to it, it's still not a reason you'd buy a spirit shield. The price of spirit shields then was purely based on their rarity.
Spirit shields dropped quickly once more started coming into the game. They've had some increases when stakers bought out all of them, but they'd instantly crash once that staker got cleaned and dumped them all onto the market. This is completely gone now that you can't stake items anymore.
There's no item sink for spirit shields and we're not gaining players at a significant rate, due to this the supply of spirit shields exceeds their demand, and the price decreases.
Regardless of their viability, it was inevitable that they dropped.