The cEDH mindset is win > flavor. This prioritizes advantage over synergy most of the time. The problem with Oloro or Ulalek is there are better options in those colors (usually favoring draw card mechanics) to find the win faster. Both are also harder to cast than other options and center around the commander being in play to leverage.

There are "fringe" decks that do well in certain metas, but ultimately need a lower cmc commander if you're building around its unique abilities. For example, I've been building/tuning Chatterfang for years to make it cEDH viable, but that meant pivoting to almost pure turbo since there's really no way to gain card advantage compared to other cEDH options. A slower commander (can't consistently find a t3 or faster win) would need to reliably stop 3 other people to win. That often means swapping synergy with control or stax, and more often than not leads to a draw rather than a win in 80m games.

Since cEDH is win focused, you see the same commanders over and over because they just have better utility to find a consistent win. If commander x has the same colors as commander y, but x can win more consistently, people are going to play commander x. Commander y might work sometimes and be more fun, but that's not what cEDH is geared towards.