I believe it's related to styles.ini, which is a single file shared by all the sets, and therefore not something a single style should make any attempt to alter (I believe the auto-downloader handles changes to styles.ini, but I don't know for sure); I used some other sets (the set folder is named in the same fashion as the rest of the L2 sets, i.e. l2_[tribe name], while the theme.nxtm is directly copy/pasted from orig_pillar) as a template and didn't really notice any obvious files within the styles that control how the style appears in the list. I'd like it to appear alongside the other L2 sets, however, so if anyone knows what needs to be done to fix it for automatic installations, I'd like to know (if you're doing it manually, you can always manually edit styles.ini).
The differences are fairly subtle: for the most part, it's orig_pillar resized to be grid compliant due to the way terrain works in L2. This grid compliance, however, can make it easier to use depending on how you build levels. I like to use the blocks as bricks, but in orig_pillar they're sized in a way that makes it so it doesn't really work unless you hide the uneven edge outside the level boundaries or awkwardly chop off the edge with erasers or other terrain. I do think the pillars are a bit awkwardly short, but it's not a huge deal. I thought about adding the larger ones from the DOS version for convenience, but ultimately I decided against it because namida indicated that resizable terrain pieces are planned for future versions.