Axis & Allies: Battle of the Bulge is definitely a wargame far down on the game-simplicity scale, but many more detailed and complex Battle of the Bulge wargames also have trouble replicating the historical "bulge" for which the battle is named. It's been a problem ever since the original Avalon Hill Battle of the Bulge game and it's been seen in just about every game since.
Many reasons have been offered for this state of affairs, but I think this consistency across time and designs comes from a more fundamental problem, I think, and that's the too-narrow view of the battlefield taken by designers.
Invariably the board starts more or less from the German starting areas (which are never threatened by the U.S.) and runs west to Liege and Namur. In the north it starts around Monschau and ends just south of Martelange.
The problem with this battlefield, especially the northern and southern limits, is that it invariably brings an "edge of the world" problem. The German player is always tempted to go that extra hex or two to the board edge to anchor his flanks. The cost in terms of diluting his main effort a little is more than balanced by not having to garrison his flanks. Even games that allow allied flank attacks rarely have provisions that are enough to discourage this tactic. In A&A:Bulge there isn't even this danger, as Allied reinforcements can only appear behind their own "front line."
In the actual event, of course, there was no edge-of-the-world. The front extended for many mile north and south and pushing in either direction would have direct consequences in weakening the German drive west, while adding no appreciable benefit, because there would always still be a "bulge."
It mystifies me why, after all this time, Battle of the Bulge game designers insist on restricting the battlefield to its historical extent. If they merely added another 10 miles of front north and south they could ensure that there would be a "bulge." Adding another 25%-33% more front would illustrate why the German attack unfolded as it did. If the German player tried to advance across that whole front he would obviously be diluting his attack beyond any hope of a breakthrough.
A&A:Bulge compounds this problem by giving the German player no real incentive to drive as far West as possible. There are 24 victory points of towns within a moderate distance of the start line and a broad front advance that reaches as far as Werbemont and Bastogne will be enough. There's no need to even get as far as Liege, let alone Huy or Namur.
This is not to suggest that the game isn't an interesting game or that the victory conditions don't result in a fair game, but they seem designed to ensure that there won't be a "Bulge" in the Battle of the Bulge on you tabletop. It would be easy to dismiss this as a flaw caused by it being just a simple, entry-level wargame, except for the fact that it's seen in just about every Bulge game ever printed.
A similar problem afflicts Gettysburg games, although on that topic at least a few designers have experimented with expanding the battlefield enough to the east and south that there's enough room to maneuver and try alternative strategies.