The price of super defence potions had declined since Jungle strykewyrms, Desert strykewyrms, and Ice strykewyrms were released, as these strykewyrms all drop super defence potions in significant numbers. It is believed that players selling these drops have driven down the price of the potions. They've since began returning to a value closer to the other super potions.
Another theory is that the release of the enhanced excalibur, which gives a defence and life points boost, has made the super defence potion redundant. However, super defence potions give a larger defence boost.