When keeping records and posting the results of an ongoing campaign, the last report is the hardest to write.
The earlier play reports have a certain momentum to them. Writing them down helps you add up the experience you need to award, keep track of the treasure they've collected, make note of the NPCs you've introduced.
But the last report carries no particular urgency. There is no "next game" that needs the report to be ready before it can start.
Too, whatever caused your campaign to end likely stands in the way as a barrier to keep you from writing. Did you lose interest in the game? Then how will you manage the interest to write? Did you become too busy, did your players drift apart? Whatever responsibilities pulled you away from the gaming table weigh heavily on your writing hand too, holding it down.
Did you start a new campaign? Your mind is there, your time spent preparing for and recording that, with none to spare looking backward.
And of course, as in all things, the longer you wait to start, the harder it gets to do so.