@ Deku
Because the Ice Climbers are always super strong. Also, the Ice Climbers in Brawl are easily one of the ten best characters in Brawl (and Melee). The Ice Climbers in Brawl force you to approach them, they can do all kinds of crazy strong moves. In Melee, they can do some crazy combos on you, eff up your L-cancelling timing, grab you to death, and do all kinds of crazy stuff.
Also, if Diddy and Dixie Kong were to team, here's what would need to happen to make them "balanced:"
1. Diddy Kong would need to weigh a lot less. Like, less than Pikachu. The same goes for Dixie Kong.
2. Diddy Kong would need to do about 150% less damage with each of his moves (ie: if a move does 6%, it now does 2%). The same goes for Dixie Kong (if she was original and in the game).
3. Both characters would need to be garbage alone.
That idea ultimately would make both of them awful characters, and not very enjoyable. That's why characters that or the same or almost the same (ie: The Ice Climbers, Plusle and Minun) work and nobody else does as an idea. It takes away from the originality and uniqueness of the two teamed characters, and makes them a bland tangent. That or their originality and diversity makes them literally cover up all of their weaknesses. Either way, you lose.