In reply to Brad Williams.

WP.com free themes will keep being submitted to the WP.org repository, but we're releasing new ones much faster than the submission pipe to WP.org allows, leading to a large backlog.

As Sarah mentioned in the article above,

The Theme Review Team added a new rule last year that theme authors can only upload one theme at a time and will have to wait for it to pass through the queue before submitting another. This severely restricts individuals and companies that are prolific theme authors, requiring them to wait months in the queue. Authors can realistically expect to only publish one or two themes per year on WordPress.org.