Party loyalty. Economy also not so bad. Her party did so well last time, they could afford to lose a lot of votes this time and still come out on top.
The second strongest party, the Socialists, split the left vote with the Greens and the Left Party, so even though Merkel's Christian Democrat lost votes they are still the strongest party in the Bundestag.
The funny thing is that the Socialists (or Social Democrats) are also Merkel's current coalition parties, so it's like the Republicans and Democrats formed a governing coalition with the opposition composed of Libertarians, Greens, Reform, Constitution, and various socialist parties.
Word is Merkel may try for a coalition with the FPD and Greens this time.
Sicherlich ist Deutschland verrückt geworden - noch einmal.