To answer without answering: it's because we don't have a clear, unambiguous definition of whether military/weapons systems questions belong here. To speculate: the majority of users of the site seem to be interested primarily in Commercial or General Aviation.
On and off-topic is far too losely defined on Aviation, leading to questions like this. If there was a Military.SE this would fit perfectly there, but until then the question to ask is "How aviation-related is this question": I can't see it, as it's been removed, buty guess from the title is that it's slightly too specific to the weapons system.
Considering that we have numerous other aircraft-specific questions, I'm not sure where we draw the line, but as an example the following Cessna 172 question is regarding fuel, which is aircraft specific in process but generally aviation related.
Then we start to get into the trickier ones - this one is VERY military and only linked to aviation in as much as "Planes are the target", which hasn't been closed
Or this one which is purely about weapons again, albeit air-launched ones, and hasn't been closed
Or even this, which is about counter-weapons systems on a commercial airliner... is this really so different from a question about weapons on a military aircraft? Yet again, this hasn't been closed
Very simply, we aren't applying these rules consistently, because we aren't quite sure what the rule is. We need a consensus on the rules, to clearly define the rules, and then to apply/enforce them correctly.
The solution: define this site as being non-military aviation, or as purely involving aviation, navigation, avionics, ATC etc.... or define it as incorporating military aspects as long as they are closely related to aviation (air-launched or anti-aircraft weapons, for exampel)