Because one of these allows you to make predictions, and the other doesn't. Saying "fire has a cause, and I'm going to call it 'phlogiston'!" doesn't tell you anything about fire, it's just a relabeling.

The hypothesis went a little deeper than that. "Flammable things contain a substance, and its release is fire" lets you make many predictions — e.g., that things will burn in vacuum, or that things burned in open air will always lose mass (this is how it was falsified).