Why can't most people remember their past lives?

Now, of course I can't remember ALL of mine, just like I can't remember every single day I've been alive, but I can certainly remember most of the noteworthy ones. Yet almost everyone I've talked to can't remember anything from any of their past lives at all, and some even go so far as to deny that they've reincarnated at all. Is it some kind of special skill or something you need to do it? I know it's not simply some sort of delusion or false memory or whatever, because I've tested it, looking up obscure historical facts that I "remember" from myself living in that period. And every time it's proven to be true, when there's no conceivable way I could have known about it beforehand.