Because they had roughly the same subject, merging them would bring back the thread to the top of the list (thus no issues with it being a few weeks old), and with things like "the photos aren't really that much of a theme" in the first post it doesn't indicate that it is a "themed set of photography" to me (or is themelessness the theme?), but rather made it look like you just got a new batch of unrelated photographs done that would thusly fit perfectly in that thread, as a sort of repository for your published photos.

It's perfectly fine with me if you open a new thread if those photographs really stand on their own, if they would've been a (like you said) themed set of photography, but to me it seemed like they did not.