Why can't obama win based on the delegate count?

According to CNN.com there are 4049 total delegates 1845 pledged to obama 1686 pledged to hilary and 19 pledged to john Edwards. That would leave 499 delegates unaccounted for. The candidate needs 2025 to win the nomination, so obama is only 180 delegates away. To get that many delegates he would only need 36% of the remaining delegates. So why are all the news channels saying Obama can't win based on the delegate count alone, it looks like he can to me.