Think of this: one day, the scientists found a little meteor that contains a lot of gold, more than the mass of gold on Earth combined. Not only do they know exactly how to harvest it but they also invent a method that allows people all around the world to participate in harvesting the meteor. Now everyone has a lot of gold, gold becomes something usual, its price drops, every grams of gold ever been stored become nothing. It stops being a store of value.
With the example above, we can agree that gold is not permanently valuable. The same logic applies for Bitcoin, its price goes up and down. Maybe someday in the future, everyone gets tired of it and stop using it, would it be a perfect store of value anymore?