As much as the Social Democrats are often seen as the political equals of the Christian Democrats in German politics (at least historically), the SPD has only had two elections with a higher percentage of the vote - 1972 and 1998. 
In 1972, Willy Brandt led the SPD to its best results of the post-war with 45.8%. Brandt was able to expand from its Ruhr and northern urban base and even had directly elected seats in the CDU/CSU-dominated south.
By the 1980s, the SPD's core of support was clustered again on the Ruhr, industrial towns and cities in the northern parts of West Germany. Here the 1983 election where Hans-Jochen Vogel lost to the CDU under Helmut Kohl by 10%
While many expected re-unification to be a boon for the SPD, the re-formed Social Democrats did poorly in the former GDR where the CDU rapidly established a dominant electoral position. Here 1990 in the first joint elections. SPD support in the East 10% lower than West.
Only in 1998 did the SPD finally break the CDU's majority and end the long tenure of Helmut Kohl. Like Brandt, Schröder was able to expand beyond the SPD's traditional base in the north and west, but also expanded the party into the East garnering 40.9% to the CDU's 35.1%
By 2009, the SPD's gains in the East had been lost to the CDU and the Left and the party was back where it had been right after reunification. In the West, it's support dropped dramatically.
In the last Federal Election, the SPD hit a low point with 20.5% failing to compete for directly elected seats outside of its traditional strongholds in the Ruhr, industrial cities and towns in the northern part of the West and some patches of Berlin.
All of this is to say that the SPD is not a natural governing party that is primed to easily retake power if they just get a charismatic leader. The party has always been at a disadvantage and required the support of a strong FDP to govern in the 1970s under Schmidt.
The SPD's challenges are long-term and structural as its traditional base of industrial workers continues to erode, it loses middle class voters to the moderated CDU and enviro conscious Greens and its one-time inroads to the East and South have failed to take hold.
