Assange has been taunting about those mails for a while, they could've released all of them at once not drop by drop with the last one being on the weekend before the elections.
This was played for maximum effect, they had them likely for months, if not longer, this is a score settling not an exercise in democracy.
To be fair though, if they released them all two years ago no one would remember it at election time. The great unwashed masses tend to have an attention span and memory that can be measured in seconds.