I am not sure I understand what the desired outcome of this post is. Its really a no win situation. Lets say a generic reason is added and when threads are closed that reason, say something like "Doesnt contribute to goals of site" or something like that, is given. Now what you have is more threads started with titles that state "How was x not a contribution? Do you guys agree?" So, in the end its circular and pointless.

Sometimes the best explanation is none at all, because this prevents follow up questions that just re-hash the same things over again.

Rampant speculation is always going to occur regardless. Like I said, a generic answer isnt going to please everyone. So, whats the real goal? Even with a concise explanation, if you disagree, then another "I disagree" post is going to be made. Spending time moderating forums for a few years and you realize that people just really want to complain. Having dumb things like Karma only adds to this problem because it incentivises making posts.