Let's just face facts, right here, right now. There wouldn't be any Bartholomew and the Oobleck if it weren't for foolishness and folly, because Foolishness + Folly = the King; and the King = the reason oobleck happens and we get ourselves one whopper of a Seussian tale.
King Derwin is blinded by his folly. He wants the only thing he can't have, and he'll stop at nothing to get it, even though he knows nothing about what the magicians are creating for him. He doesn't stop to think about any dire consequences, let alone dream that they might affect him. That's foolishness. Foolishness to support his follies. And not the nice and Broadway kind, either.
Q: Why does the King want to change the weather?
A: Because he's a control freak. And stubborn, to boot.
Q: What in the world is oobleck?
A: Non-edible green goop.
Q: Why doesn't the King get worried that oobleck is blanketing his kingdom?
A: Because he's one foolish guy.