@The Frowning Pony
Mostly it's only done if the description violates some rule (in which case a history for it defeats the point) or clearly violates the spirit of the description field (is just the uploader making a comment about the image, often disparaging).
Tags can violate rules and be disparaging, but this information isn't removed from the change history. Considering violating descriptions would only be displayed on a separate page most people don't care about, I don't see any problems with preserving changes history of descriptions too.
By the way, I don't understand why descriptions can only be changed by the uploader. Copying descriptions from the artist's website, cross-linking related pictures and other useful activities can be performed by many users.