Mostly because more people voted against her and her policies than voted for her or for candidate aligned with her.
Sort of like the US elections of 2000 and 2004, where more people voted Democratic at all levels of government than voted Republican. Unfortunately, here, we don't do coalitions since we only have the two parties (and whatever crackpot decides to run a vanity campaign).
Interesting analysis here.
Angela Merkel's Christian Democrats and Gerhard Schröder's Social Democrats both claimed victory for their parties in Sunday's closely fought election, but with 35.2 and 34.3 percent respectively neither party received a clear mandate from the electorate. But a total of 51.1 percent cast ballots for the three parties to the left of the center, the SPD, the Green party and the newly formed Left Party. Only 43.9 percent voted for the conservatives and the free-market liberal Free Democrats (FDP).Which is why Schroeder can still lay claim to being the leader of any coalition government, since his is the dominant party in the majority vote.
But as Germany desperately weighs the different possibilities for a governing coalition, the SPD, Greens and Left Party have all said there's no way the three of them will work together. The Social Democrats and the Greens have stressed they won't work with the Left Party, which, for its part, has said it won't tolerate the other two in a minority government.
Effectively, this weakens Germany as a world power, since Schroeder does not have the added mantle of being the top dog (he lost the election, after all), and therefore must consult his coalition partners on any major decisions.
Merkel, by contrast, would not have this problem if she can swing a conservative coalition, since she is the top dog on her side of the fence.
*whew* And who says politics is boring?