I thought it was due to the relative newness and hence, paucity of ratings to average out into a score, but it wouldn't have made sense since Pause Ahead and Loot Hero have been around for a while, and they had ratings.
There is an option to vote up or down on these games.The mmo's didn't have anyway for people to rare them. The clock is a pending symbol normally used on a few other places on the net.
You can rate MMOs now (up/down vote). The clock is a "timer" and means that there aren't enough ratings to show what the rating is -- a new feature. Since you couldn't rate MMOs before, all of them will probably have this clock icon, and any new games -- or games that people just haven't rated much, for whatever reason.
If the timer wasn't there I should think the rating would go crazy with so little plays, how many plays does a game need to get before the timer is removed and the average rating is displayed?
I feel the reason to hold off on displaying the ratings right away is two fold.
1.)Law of large numbers This law starts that after performing an experiment a repeated number of times, the average result should be close to the expected value.
How this applies is, when a game first comes out the number of ratings isn't large enough and the average rating shown will jump around a lot. Also, the average ratings shown during this time do not accurately reflect the quality of the game.
2.)Herd mentality Herd mentality basically describes "how people are influenced by their peers to adopt to certain behaviors". (quoted from Wikipedia)
How this applies is, when a game first comes out it may have an inaccurate rating displayed (due to reason 1, not enough ratings are in). If the game is favoring toward a low score, players will see that and be preconditioned to give the game a lower score than they normally would have. The effect can work in the reverse manner as well, with players giving higher ratings than they would have. (This related article helps to explain the phenomenon)
Herd mentality's influence on player rating may not be a lot, some people not affected at all, but it can still add up. My guess is that it can cause a player to rate a game one point higher or lower than they normally would have, just to have their rating be closer to the average rating.
Conclusion: Holding off on displaying the ratings prevents the early, less accurate, rating averages from being displayed and it prevents the first few votes from having an effect on the rest of the votes cast and, ultimately, the final rating of the game.