Remember that in order to break the defender, the attacker has to score DOUBLE the number of hits. Statistically that means that on average, he needs double the number of dice.
Assuming the Line did a really crappy job shooting, and nobody else was shooting at either column, let's say the attackers only suffered 1 DISR between them, going in. That means they get 15 dice, versus 8 for the Defender. (Assuming all other things being equal.) Not quite enough.
If you -don't- double the defender, then the combat is Inconclusive, meaning that he suffers 1DISR and falls back. Worse - if the defender manages to equal or beat your number of hits, then EACH attacking unit suffers 1 DISR and falls back, which can get really ugly really fast.
We're making a lot of assumptions here, too:
1. That the defender didn't choose to Fall Back, as a Reaction
2. That nobody else managed to shoot at those two attack columns over the past couple of turns as they got close
3. That nobody has any advantages for SK, Esprit, terrain, etc.
One of the things I noticed over and over again when observing the many playtest games was that new players initially think: "Wow, this game really favors the attacker!"
And after two or three games they realize: "Wow, it's really hard for the attacker to win!" (There's a reason the army lists give extra units to the attacking side.) A defender in Line has the advantage that he's covering more front with fewer units and thus probably has a reserve nearby... not to mention that his artillery is a lot more useful than the attacker's, which is often masked by his own advancing columns. If the attacker wants to shoot before he goes in, then the turn sequence means that he has to leave himself within musketry range in the defender's upcoming Reaction Phase, and thus he'll get shot *twice* (in that turn, and in the next turn when he actually charges), while only being able to shoot at the defender once. A clever defender can also retrograde move a bit, thus prolonging the time he'll get to shoot at the attacker.
As you play more you'll find a number of advantages to being on the defense, and ways to torment your opponent.