So, the thing with the forums here is that they aren't based on BBCode but instead based on a loose interpretation of the Markdown text "markup" language. There are some things in Markdown that are purposefully left out of the boards (who needs tables anyway). As far as I'm aware, there's simply no Markdown support for spoilers - meaning that not only would it have to be coded in-house and added to the forum software, but to avoid breaking the current posts that are formatted with Markdown, it'd have to be 100% non-conflicting with Markdown. It's sadly a lot more complicated than just flipping a switch and allowing them, or I certainly would've done that the day I became an arbiter/advisor :P
Hopefully this answers your questions? If not, feel free to shoot me a reply and I'll try and help however I can :)