The problem with allowing low scores is that, essentially, it just makes this competition show one thing: how many people disliked the game.
Practically all the games in the competition average over 8.0, with the majority averaging over 8.5, and a large chunk averaging over 9. An 8 is practically a "I bought this game and I liked it" in terms of this competition.
Allowing very low scores causes a problem. If someone absolutely loves the game, and think it's the best game of the generation, they might give it 9.5, or 9.7. Someone who dislikes it can give it 5.2, or 3.4, or some other very low score. Now, why is that a problem?
If you say that 8.0 is the average (it seems to be), then a person who loves it can say that it's up to 2 points bette than average, whereas a person who dislikes it can say that it's up to 8 points worse than the average. In essence, you now have a ruined competition.
To make an example: Uncharted 2 had around 30 votes for it, and got an average of 9.55. Now, say that some random person entered and gave it a 3.0. It now has an average of 9.33.
A single vote can have a huge negative impact, but barely any positive impact. Another perfect 10 would just bump the average up to 9.36.
Essentially, your allowal of low scores has made the game useless for comparisons, all it shows is which games had a few people give it a rubbish score.
Now, if you had either forced 6-10, removed the top 10% and bottom 10%, or used the median, you would have had a somewhat viable comparison, instead of what it is now.
