Straight from the JSON Feed homepage:
We — Manton Reece and Brent Simmons — have noticed that JSON has become the developers’ choice for APIs, and that developers will often go out of their way to avoid XML. JSON is simpler to read and write, and it’s less prone to bugs.
See the spec. It’s at version 1, which may be the only version ever needed. If future versions are needed, version 1 feeds will still be valid feeds.