Clamp isn't used to sweep, it's used to control the battle by abusing free switches and by covering anything that switches in. Clamp and explosion make cloyster one of the best switch-ins to a chansey ice beam.
Slash is only better than goldom when boosted, otherwise it's inferior. Grounds in OU are hit and run pokemon, you get them in on something, hit something on the switch, then switch out. It doesn't really matter that slash can take specials hits better because it can't stay in on specials anyway.
If you're implying that slash is way better than kingler, then you're wrong. The match-ups that matter most to SDers are the pokemon you're most likely to see end-game, and kingler has a better match up against tauros. Tauros can't hit kingler for SE damage, whereas kingler 2HKOS with chammer. Kingler is much more of a threat unboosted, having a good match up on snorlax, 3HKOing with chammer.
Twave immunity is nice, but the idea behind SDing is that the opponent's team should be so crippled beforehand that they won't get many attacks off to begin with. Kingler has a better chance at setting up because it is much more of a threat unboosted, meaning it can force more switches.
Victreebell doesn't have the firepower that kingler does, but it hits pokemon like starmie and SE,, and has status.