Well, if the sword was symbolic and Soon was merely making Incorporeal Touch Attacks, that would explain it. See, Incorp Touch Attacks don't get a Str bonus to damage, because they're not solid. You can hack the crap out of a living target, but breaking something durable and unliving isn't so easy.
What are you talking about? Look:
(1) Soon kills Xykon and Redcloak
(2) 1d10 days later, Xykon pops back up
(3) Xykon has Redcloak reanimated as an intelligent undead (or The Dark One does)
(4) Even if #3 can't occur (Xykon may not have access to such spells), Xykon still exists...