Because at least 12 different approvers looked at it and decided they weren't interested. That's it. It literally takes only one approver to decide a post is good enough for the gallery. Barring cases where a post breaks the rules or is so egregiously bad that a moderator deletes it unilaterally, the only way a post gets deleted is if the approvers are effectively unanimous in not liking a post enough to approve it. See about:mod_queue.
As for the specific reasoning behind why none of the approvers like this, they aren't required to justify their decisions; several hundred posts get submitted through the modqueue daily and no volunteer wants to submit detailed explanations for every single post they look at. You could always ask in topic #7934 and maybe you'll get some feedback from an approver, but no one is under any obligation to respond.
