You'd be surprised at how different votes and flags are in effect. The upvoting system is a lot more broken than the flagging system is. People tend to upvote as a reflexive "me like" instead of a reflective "this is interesting". (That's not a criticism—it's simply the chemical reaction of the voting mechanism and human nature.) Flagging is much more reflective in practice. You can think of vouches as an experiment in seeing whether up-flagging can contribute as much value to HN as down-flagging has.