From RT website
The movie hasn't even been widely released and the score has fluctuated, so I see why they would wait until the movie receives 100 or 200 reviews.
http://www.rottentomatoes.com/help_desk/licensing/Rotten Tomatoes awards the Certified Fresh accolade to theatrical releases reviewed by 40 or more critics (including 5 Top Critics) with a steady score of 75% or higher on the Tomatometer. A film remains Certified Fresh unless its Tomatometer falls below 70%. Reserved for the bestreviewed films, the Certified Fresh accolade constitutes a seal of approval, synonymous with quality.
The movie hasn't even been widely released and the score has fluctuated, so I see why they would wait until the movie receives 100 or 200 reviews.