I believe I'm fairly good at explaining, partly because I'm good at laying things out logically, but mostly because I'm actually able to believe that other people don't understand things just because I do. (Let me know if you think I'm kidding myself about this.)
I believe most people are fairly bad at explaining because they don't imagine not understanding the same things they understand.
I believe, on no strong evidence, that people who are bad at explaining would find it difficult to learn how to be good at explaining. In any case, I can't see how to explain the fundamental leap of really believing that people don't understand something-- they aren't just being perverse.
I realize there might be some contradictions between the second paragraph and the fourth.