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Because Ash's adventures build on each other in some aspects but fail to do so in other aspects. In the Pokemon anime, when Ash catches a new Pokemon or gets a new badge, he retains it and moves on to the next experience. As we watch Ash progress on the path to his goal, this series of experiences becomes the character's history. Ash has built up an impressive history over hundreds of episodes, yet he never ages or retains the lessons he learns; that inconsistency is what's frustrating. The show is episodic in a way that Ash should be growing, but he never does.
Spongebob, on the other hand, has no history. (for the most part) While Ash in Kanto and Ash in Sinnoh will have drastically different Pokemon and badges, Spongebob changes little from episode to episode. He has his misadventure, goes to bed, and wakes up next episode to have another misadventure completely unrelated to the first one. You could watch them in any order and it wouldn't matter.
tl;dr Pokemon has the illusion of growth without any of the payoff, while Spongebob uses a format that doesn't promise or deliver any growth
