On the Debrett's page Cheryl linked in her post below, it said that it used to be published annually.

I would be surprised if they were ever free. I see that it costs 295 pounds for a copy of the book now (!!) so I can't imagine they were giving them away before.

Perhaps the fact that Sir Walter just wrote it in is an indicator of thei financial straits at this time? As in, they couldn't afford to get a new one every year? The book does not change all that much from year to year, so it's somewhat of an unnessary expense to basically keep buying the same book over and over again, no matter how fond you are of it.

Or, perhaps the marriage of his third daughter to a gentleman farmer, while important enough to add by hand, did not warrant purchasing the book in Sir Walter's opinion? The last really 'important' matter that happened would be Lady Elliot's death. If Elizabeth were to marry someone she and her father felt to be worthy of her, I'm sure they'd get the latest edition.