It may actually turn out to be a historic election. For the first time since their conception the FDP may not be in the national parliament. It is amusing that the CDU (conservatives) absorbed up most (~8%) of the market liberals, which were their coalition partners for the current government. Merkel is just not a great choice to team up with.
As for an explanation to the success of the CDU, i would guesstimate several factors.
- Poor leadership figures for the other parties
- Very broad agenda of the CDU/CSU (reaching from right to left)
- Merkel has coined the term "presidential hesitation" in german political science; she does not lead, she waits for a majority opinion to form and then follows that, not always, but most often
- Merkel has been able to keep her own party in check, potential competition miracourasly experiences scandals that end their political careers
- Leadership conflicts within other parties
- The german left is also splintered into at least 3 parties - SPD, Greens, The Left
- The creation of the AFD (Alternative for Germany) could be seen as a problem for the CDU as they are conservative competition, but what it really signifies (particularly considering the poll results) is that there has been an overall shift towards the right. In the long run the CDU will probably profit from the AFD. Once they have been sufficiently stigmatized as nationalistic extremists, their progress will stagnate, voters will become frustrated and turn to the next best option, which is the CDU (or they wont vote, which is also no harm).
Overall Merkel wins, because the opposition is weak. She does not have to show profile and take risks, because there is no serious competition. On the other hand anyone hoping to compete with her has to gamble to catch up with her.