I know it's popular to call conservatives dumb and while I don't like to beat dead horses, I really think the explanation for this is that they're dumb. The illiterate form of dumb, to be precise. The caps are a way of adding emphasis - which is something that can also be done by phrasing, word choice, sentence structure, and so on. But those techniques would require a beyond 4th grade level of writing and reading ability, so they do not succeed in the conservative communication marketplace.
I will add a disclaimer, my beloved Nietzsche also does all caps words sometimes, but I believe that since this is alongside his impressive eloquence, it is clearly not a sign of stupidity in that context. Likewise, it is not a sign of stupidity in many other contexts. That is to say:
Using that style of communication does not always make it a safer bet that someone is stupid, but being stupid does make it a safer bet that they use that style of communication.