Currently on prison, the most expensive spawner you can buy is a guardian spawner. As you may know, guardians drop Prismarine Shards, Raw Fish, and Prismarine Crystals. However, you are unable to sell Raw Fish and Prismarine Crystals, which reduces the profit being made when using them, and ultimately clogs sell chests. Considering that the skeleton spawner also has three different obtainable items that all may be sold, it makes the cheaper spawner more profitable. It only makes sense to allow the sale of Raw Fish and Prismarine Crystals also.