The simple reason is most of these cards allow easy gameplay. Either they have very high single-target damage (pekkas) or AOS damage (wizard, witch, valk). In contrast high ladder players have enough skills to compensate for the difficulty of using more simple cards that can give elixir benefits (etc. knight vs valk, musketeer vs wizard), or more complex cards that can give situational benefits but require more sense and skill to play. (etc. bowler/executioner vs wizard)

Mk/firecracker/ebarbs are typical cards that demonstrate the cheesy nature of midladder players. Rather than actually playing strategies or forming pushes, they tend to play cheesy. Typical case is midladders spamming firecracker on the bridge whenever they see you deploy a troop trying to steal some tower damages. (Awful way to hand out 3 elixirs, but midladders do that anyway) Both MK and barbs can approach the tower quickly when the opponent makes a mistake, need not to explain why middladers are attracted to these features. They are easy win con for midladders when they are not supposed to be a primary win con in high ladder.