When you set-mine, you're assuming you'll be getting a stack often enough to justify the times when you hit a set.
In my experience, players are playing more hands than in full ring, so its less likely that they'll have a hand to stack off with when you hit your set. So in 6max, you need better set mining odds to do it profitably. There's also more 3-betting in 6max, so its just harder to get to a flop for cheap.
However, once players start getting aggro on the flop, they fold less on later streets. So calling with draws becomes more profitable.
It just has to do with where the "implied odds play" is happening. With a set mine, you're calling preflop & hoping to hit a fit or fold hand on the flop. With calling with a draw, you're calling on the flop hoping to hit a fit or fold hand on the turn/river. So in 6 max, peeling the turn/riv is more profitable.
I hope that made some kind of sense.
Okay so how do we know when to 3-bet? when should we do it and why shoould we do it?? or to whom to we do it to?
Oh boy. Um... you could write a lot about 3-betting light. So I'll just tell you the reasons we 3-bet are:
1) It counter acts our positional disadvantage. So if we're raised preflop by a late position player, and we're in the blinds, then we 3-bet to try and avoid playing the hand out of position.
2) Players open light. Since the blinds hit you more often, and there's less players, people play more hands. They make more steals, and they raise with weaker hands. So we can 3-bet more hands for value, since their raising range is wider.
3) Getting your blinds stolen is a bit annoying, and you should defend them once in a while if a villain is getting ultra-abusive. But this isn't that big of a reason.