It's not surprising that it is so rare for a team to win back to back championships in baseball. Due to the randomness in baseball results, it is mathematically very unlikely that a team that makes the playoffs two years will win both years. If we assume 100% randomness, it's less than a 2% chance. It's probably not 100% random, so maybe a 5% chance. And of course, a team has to actually make the playoffs twice in order to have any chance of repeating.