And why not announce the ones they have gone through?
Because, as a business, you want to keep your ducks in a row, and keep your message clear and simple. I, personally, would prefer if they just officially said, "Yes, we have gotten some archival material, and we are in the process of evaluating it," and leave it at that. I can understand, though, that they do not want to make a moving target out of the list of stuff they have. Let them review, let them actually do the restoration, review again, and then tell us what is there. The last thing they need is to promise things, and then not be able to deliver.
Sadly, it was not just Doctor Who that was destroyed--BBC destroyed almost ALL of their archives. Tons of recorded shows from the 50s, 60s, and 70s were lost.
From a historical perspective, yes, it is sad. But, honestly, I don't much care about the rest.
However, this may present another reason why they might delay in announcing things - initial rumors were that they had gotten a huge chunk of material, and Doctor Who was only part of the batch. The term "three tons" was used. If it was not cataloged properly before they got it, they may not know what they have, and may have to wade through all of it before they can say. Then, it is more than just doing a focused review of Doctor Who serials. If the collection is truly massive, the person-hours need to review and catalog becomes an issue.
And, honestly, from a business standpoint, holding off for the 50th makes about three tons of sense. That's not a "stunt", that's just good marketing.