Why we see the same characters in KOF tournaments

The tier list in KOF13 is very "scrunched". Most good players would classify the characters into B, A, and possibly S tiers, though personally I'd just rate from B to A+. Even top players will agree that the mid and low tier characters are not "bad", definitely not. Rather, due to the universal nature of the game's systems, the characters that I would rate at A+ (Karate, Kim, Benimaru, EX Iori, Chin) simply have tiny advantages over the rest of cast that end up mattering more at the world-class level of play.

For example, Karate's damage output and zoning capabilities are above average. You could pick Takuma and get a damage boost on clean hits, but you'd be giving up a few tools (QCF+P, HCB+K, air HCF+K, jump cancel mixups). Most top level players have decided that the tiny damage difference is negligible and would rather have the extra options/gimmicks, and a similar logic can be employed to understand why Kim, Benimaru, Chin, and EX Iori are so strong and popular.

As MadKof has shown (as King vs Frionel's Karate, as Daimon vs Fox's entire team), fundamental skills and good reads can go a long way towards negating the strengths of the top tier characters. The players that have reached that level feel that they would rather stick to the effective strategies that they are comfortable with rather than take a risk on more specialized characters. The mid and low tier characters in KOF13 are still highly competitive, but they might have to land one more hit or might have to go without a meterless uppercut, and so the best players in the world would rather not enter a match with that weakness.