Here's how it works according to the Wiki.
CPU count = current player count. This fluxuates as people quit and start the games.
Red bar = total time spent playing these games. The bar can only go forward, not backwards.
The more people playing the games, the faster the red bar will increase, as obviously the more hours are being put towards that game. The less people are playing a game, the less hours are going towards that bar, and thus the slower it will increase.
The kicker is that the speed at which the bar increase is shaped to the amount of people who own that game. Not playing, but own. The more people that own a game, the more hours are required to fill the bar to completion. The less people who own the game, the quicker it will be to fill the red bar.
Not many people own Kick It!, so it will be easy to fill. Plus, people are focusing on playing it, so it's not only one of the easiest games to fill but also has the largest current player focus. Hence why it will be done soon.
On the downside games like Killing Floor likely have a lot of owners, and thus will require more people playing to increase the bar.