Why?
Why it has a very good rating and it's not in the best 250 films of IMDb? Because it's from Argentina?
shareNo. It's because to be in the top 250 (or the bottom 100) a film needs to have a minimum of votes, too. At this point, the minimum required is 1300 votes. Otherwise, the best film ever could just be one that happens to gather five 10's. The average would also be 10, but it would not necessarily be representative of what most people here think.
Besides, I'm not sure, but I think that not every voter's vote is considered in the same way. I think there are some categories of voters, and only some voters' opinions are taken into consideration to make the top 250 and the bottom 100.
See at the bottom of this page: http://imdb.com/chart/top