I thought I would reply here because maybe my experience may give you some possible other explanations, in addition to what DM and others posted. I used to try making a lot of original decks. I have made some combos which I had never seen similar before. Of course, those that did better, well, I have seen the ideas used since. Now, does this mean that those type of decks never existed before, no, but having not seem them in the prior years, they certainly weren't common. Problem was, my more original decks did not get me any more thumbs up, and of course, they usually lost more often than did my better decks. And in Platinum, a few loses and you are out (even if you have more wins than losses). So decks such as those are less likely to stay in there long. And why be original if there is no benefit to do so (few thumbs up to help keep it active).
But what really got to me was that most EVERYONE was playing the same few decks to farm Platinum, i.e., PDIALS. No originality what so ever. And few ways for my decks to win. And no, I do NOT want to give away the win to those farming; I actually WANT my decks to win. Opponents should have to work at it to win. Do I play any of those cookie cutter decks when I farm Platinum, no, but most people do. Besides this, I happened to become more busy and have had less time to spend being original. The above is the frustration which made me adjust my decks to having some which are more likely to win. Are they exactly the same as others, not likely. With the exception of decks which are meant to outlast such as Light decks, most people don't use 60 card decks, while I mostly do. This means that although I may have some of the strategies of of some others, I have additional cards, which means that the play is more likely varied each time. Have I ever entered those decks you quoted? Some of those you quoted I have never made, but I have others, i.e., mono Darkness. But I have never ever made a PDials type for the AI to play. But what is one to do if one is given five Black Dragon to play? Like I said, I usually use 60 cards, so I have some extra cards that others may not use, but it will be what you would consider as a similar idea. Do I ever play mono Aether decks? Definitely, but seldom using a stack of Dimensional Shields (unless those were the cards provided). Etc. I actually like using varied strategies within a deck.
Like many others, I would love to get a #1 deck, but since I am still not doing exactly what most top decks do, even though I have many decks which do well, I still haven't placed in the top ten.
Ghaladh, this is how I feel: When the majority of the people use original, or at least much more varied decks to farm Platinum, I would be much more inclined to post more original ones again myself. Think about that! (I actually would very much appreciate the change.)