For the sake of the TV show, they do not want Ash to defeat the elite four, at least not at his current age. He has not beaten any of them. The anime and the games have two separate standards, so being an elite four in the anime seems to take years of training, even more than Ash has put in. Trainers like Flint have been training hard since they were very little kids, and some of them have only been admitted for short periods of time, so it's to help demonstrate just how powerful the elite four is. If ash beats one of them, then Ash will be held to that standard for the rest of their series (and you'll hear endless, "but Ash beat an elite four member! how did he lose to ____? What a noob," due to people not realizing the importance of plot devices.)