The problem with theorycrafting is that it doesn't really fit the Stack Exchange creed of "ask questions, get answers". Theorycrafting is more like "ask question, calculate answer, get feedback on answer, try something new, get feedback on new, try something different, get feedback on different, agree on a model, inject model into simulation tool, fix bug with implementation of model in simulation tool,..." It's something that's not really well suited for a Q&A model like Arqade. It's something more suited for a forum or a chatroom.
In addition, theorycrafting is an ever changing subject. Players are rebalanced every week, bosses are regularly hotfixed for bugs, and then there are the major and minor patches that require YET MORE changes to the model. Arqade doesn't really like questions that are that volatile in nature.
Finally, theorycrafting is something that's limited to a VERY small subsection of the community. Out of the millions of WoW players, I estimate that less than 100 are actively working on theorycrafting new and updated models. And while many players are using the FRUITS of theorycrafting (as in guides, simulation tools and inaccurate DPS comparisons), only a fraction of players has the right mentality for answering questions about theorycrafting, and I doubt they're active on this site.
To clarify: Theorycrafting questions themselves can be on topic. However, the normal process of the community asking how the TCer comes to a certain conclusion and pointing out flaws in the methodology is less suited, because Stack Exchange inherently doesn't like people discussing things in details.