Here's why:
* Describable means "capable of being described".
* The prefix "de-" often indicates separation or removal.
* In this case, "describable" suggests the act of removing something from obscurity by providing a description.
While "de-" isn't the only prefix that can be used with "scribe" (e.g., "subscribe," "prescribe"), it is the most relevant for the meaning of "describable."