I wouldn't say that SBM has just sat back and allowed Shadow to take their villages; I think that's a bit of an unfair generalization. To make a poor analogy: when the Spanish conquistadores conquered the Inca, would you characterize the Inca as sitting back and allowing the Spaniards to simply take them over? Or was it really just a matter of overwhelming superiority on the part of the Spaniards?
Instead, I think this world has seen a classic endgame scenario. (Certainly not classic to TW, being the first to my knowledge to progress to this point, but classic to most any multi-player strategy game.) The opening phase is an initial period of build-up and relative chaos. Some players rise quickly, some are quickly disappointed and quit, some just hang around and try to learn. Eventually, strong alliances are formed - this is the second phase. These alliances are fluid, and the balance of power shifts among the several alliances. Most TW worlds are still in this phase; it's the real meat of the game. At some point, one alliance tips the scales and becomes the de facto superpower, forcing all others onto the defensive. Frequently, this results in that superpower fracturing for one reason or another, cycling the game back into the second phase...but on occasion, it remains cohesive, moving the game into the terminal phase. In TW, this means cleanup of all remaining resistance. This game exhibits what I've heard termed as the "runaway leader" problem - meaning, once a player (or cadre of players) gets a certain point ahead of the others, it is impossible to catch them; merely by being the leader, it increases the chances of remaining the leader. (I would also say that the runaway leader problem really only exists in TW once the third phase has started, and even then, on most worlds, player egos force a recycling into phase two, but that's a discussion for anotehr time and place.)
I posit that phase three started here on W27 once the major players of [IRON] became disheartened and quit; that was when any real resistance to Shadow became decreasingly possible. The fact that Shadow has not fallen to internal power struggles or boredom, like so many other older worlds, is what sealed the deal.
It's not that SBM has just sat back and given villages away; it's that most have realized that there is nothing to be done about it. Picking on smaller tribes is likely seen as the one thing that can be done. Postponing the inevitable, surely, but still something to do while waiting.