Code complexity is about all, I would expect. Pretty much no *chan type server uses anything like that other than at the section overview level, they're all extremely simple (though Enclaved would probably beg to differ given his rant about having to disentangle the original spaghetti code in order to fix it) PHP/MySQL type affairs.
SELECT FROM TABLE "artwork" WHERE $THREAD = "1429" and all that.
Heck it might even be a simpler filesystem based setup given how the "forum" and "thread" addresses are given... e.g. for this thread, gurochan.cx/dis/res/6907.html ... this is thread #6907, and it's in Discussion, so each new post regenerates the code of page file 6907.html under folder /dis/res on the gurochan server. Loading the page then requires nothing more than reading the html, and any images inlined to it (which are also stored in a big flat pile inside /dis/res, and found by searching their numerical filename). Doesn't even separate threads into their own folders... I mean, I'd expected this to be more index.html in /dis/res/6907, but that's evidently not the case (or at least, what we can see from the client side doesn't reflect that; it might be like that in reality with the server-side software doing some live translation between the apparent and actual filesystems).
Very messy, but still entirely effective, and about as simple as it's possible to make. Writing the files so that they cut each page into sections would require adding an extra level of intelligence to the file authoring code to judge where to split them up and providing the nav links, and either making multiple .html files for each page as it grew (not the most difficult, admittedly, as just adding an underscore and then a pair of hex characters would allow 256 separate pages), or twiddling the CSS to fake that (I've no idea myself how you'd make that work, but presume it's doable; however, your browser would still have to load the entire html file even if not the pictures, and in some text-heavy threads the data load of the text and html code can outweigh that of the thumbnailed images).
Or in other words, if you're that bothered, contact the admin and offer to set it up for him, I'm sure he'd be delighted at not having to fuck with the board code any further in order to provide a change whose benefit is somewhat marginal unless you're stuck on 56k or using an ancient Celeron with very little RAM.