Sorry for the late reply, but I would presume it is perhaps something on Steam's side. I honestly don't know either, but some games just tend to get broken that way.

Additionally, even quite a few of the games I have that I've owned from Steam no longer have a store page as it stopped being distributed and projects get cancelled (for a number of various reasons unrelated to Steam). As such, despite the occassional ability to launch the game from Steam, in this case, the image (or video) would just simply default to the main store page because of broken links. If you also get a "cracked screen" default image at some point, that is a clue that it's a broken link, because that image is AA's broken link default (ya know, instead of a 404).

To the best of your ability you may be able to replace it with a youtube video or a video off another site if possible. I can't make any guarantees anyone has archived them though.