A couple of reasons come to mind:
- Very few of the best, most winning 6-max players open limp and the very few that do are ridiculously good post flop players. So, evidence would suggest that open limping is a more difficult way to win.
- Limping generally narrows your range to some very specific medium strength hands. It is very difficult to play out of position when your oponents have a very good idea of the kind of hand you have. In FR you might open limp TT in EP. In 6-max you absolutely should be raising that hand from every position.
- There are no “EP” seats in 6-max. Most of the limping you see in full ring is in EP because it is so much more likely that a hand will show up behind you. In 6-max, you might open limp from UTG or UTG+1, but there are only 2 or 3 seats behind you that could show up with a hand. You are better off reducing the number of players in the pot with middle strength hands rather than end up heads up to someone with posiiton. The current style of 6-max play is to aggresively isolate limpers. If you limp 66 and a guy on the button raises with XX, he's going to win the pot a lot with the worst hand and will rarely get stacked when you make a set because his range is so wide that he won't pay you off with most of his holdings. Imagine he raises you with J7s and the flop comes AK6. You get his c-bet. If the flop comes QT4 he's going to take you off the best hand. Even if he has Ax, you are unlikely to stack him because he's going to check back the flop and keep the pot small.
- In the CO and BTN, it is better to just raise. Limping on the button and to a lesser extent the CO is muting your positional advantage. You want to play bigger pots in position regardless of your holding. In full ring, there is just less opportunity for this as so many moer pots are raised before they get to those seats.
- 6 max is about playing more marginal holdings and much of playing marginal holdings is taking the initiative and seizing position. If you look at the degenerate case of HU, the best HU players are raising 80-100% of their buttons. It becomes a battle of wills as to who will play their marginal holdings best and that starts with aggressively playing in position.