At the time of writing this, Skylanders as a videogame property has been dead for the last 3 years, with no new games announced. With poor sales, Skylander Imaginators sent Activision a strong message that the "toys to life" market was exhausted and could not support new games. Of course, Skylander contributed to that exhaustion when it introduced loot crates in toy form : Mystery Chest.
Skylanders Imaginator introduced the feature to build custom Skylander characters to join your team. To store said Skylanders, you needed to buy Imagination Crystal, another type of toy. To create a Skylander of each type, you would need at least 10 crystals, each of them as almost expensive as a regular Skylander character.
With mystery packs, players could collect creation pieces (accessories) to build, enhance and decorate their characters. Each pack would a plastic chest, which contain 5 creation piece of different rarity, depending on the color of the chest. Gold chest were especially rare, but contained epic pieces of creation. To determine the content of the chest, you would need to scan it using your Skylander portals. Blue chest were distributed with special expansions, an contained 80 predetermined creation pieces.
Mystery chests were an attempt to move digital loot crates to the physical world. However, compared to their digital counterparts, they require material to produce, and physical space in store shelves. These two factors substantially increased their cost, thus decreasing their value. Most players would prefer buying a Skylander character they could play with instead of a couple of plastic chest.
Hence, mystery chest were too expensive to be good loot crates, and too boring to be good toys. Thus, they stayed on the shelves until being sold at a discount to the most devoted fans.