If you click on "more info" in the side bar, you will see why:
Specifically, the reason that it is 29% is because you only have 29% of the necessary users with at least 200 rep on another SE site. The % Commitment is always the lowest of the 3 shown values, and the site won't graduate to the private beta phase until all 3 criteria are at 100%.
The Startups proposal that you are comparing too on the other hand, has a much different view. It is only missing a few committers (19 of the 200 to be exact), which is why it is at 90%.