The reason is crazier than you may think:
Sometimes there's a woman
I won't say hero because what's a hero
but sometimes there's a woman
and I'm talking about Merkel here
sometimes there's a woman ... well ...
she's the woman for her time and place
she fits right in there
and that's Merkel in Berlin.
Adapted from The Big Lebowski
The decision to phase out nuclear power faster than originally planned was essentially the initiative and responsibility of the chancellor at that time, Angela Merkel, after the Fukushima accident.
Mrs. Merkel holds a PhD in quantum chemistry. It is unsurprising that a scientist comes to a different risk assessment than the average politician. Apparently her assessment was that the risk overrode all other considerations.
Here is the reporting about Merkel's decision from the usually well-informed and connected Spiegel magazine1 from April 4, 2011, 23 days after the disaster. It may be behind a pay wall for non-subscribers. The English translation follows below. Emphasis and square brackets by me.
Jahrzehntelang hatte ihr [Merkel] der wissenschaftliche Verstand einer Quantenchemikerin gesagt: Nach allen Regeln der Wahrscheinlichkeit ist Kernkraft eine vertretbare Energie. Die deutsche Angst davor hielt sie für irrational.
»Die Deutschen haben kein Verhältnis zur Wahrscheinlichkeit«, hatte sie seinerzeit als Umweltministerin in der Atomdebatte wiederholt geklagt. Nun dauerte es nur wenige Stunden, um Merkels Verhältnis zur Wahrscheinlichkeit einer atomaren Katastrophe umzustürzen.
Die Kanzlerin war am frühen Nachmittag des 11. März auf dem Weg zum Europäischen Rat in Berlin-Tegel ins Flugzeug gestiegen, mit der letzten Meldung versorgt, dass ein fürchterlicher Tsunami Japan überschwemmt habe. Vier Tote waren offiziell bestätigt, als der Airbus abhob. In Brüssel gelandet, schaltete Merkel ihr iPad ein; während der Fahrt in die Stadt verfolgte sie die Meldungen.
Um 15.30 Uhr mitteleuropäischer Zeit wurde gemeldet, dass Japan den atomaren Notstand ausgerufen hatte. Im Tagungszentrum der EU-Regierungschefs verfolgte sie, wann immer es ging, die Fernsehbilder aus Japan. Es war etwas eingetreten, das Merkel für nicht vorstellbar gehalten hatte. Sie ließ sich nichts anmerken und absolvierte die Sitzung äußerlich ungerührt. Aber für sich, ganz im Stillen, hatte sie eine Entscheidung getroffen.
»Das war's!«, sagte sie am nächsten Morgen, als sie mit ihrem Büro die Lage erörterte. In Fukushima ging für die deutsche Kanzlerin das atomare Zeitalter zu Ende.
English translation mine:
For decades, her [Merkel's] scientific mind of a quantum chemist had told her: According to all rules of probability, nuclear power is a defensible energy source. She considered the German nuclear angst irrational.
"The Germans have no connection to probabilities", she had complained repeatedly in the nuclear debate, back when she was secretary of the environment. Now a few hours was all it took to topple her own connection to the probabilities of a nuclear catastrophe.
The chancellor had boarded her airplane in the early afternoon of March 11 in Berlin Tegel, supplied with the news that a terrible tsunami had inundated Japan. Four fatalities were officially confirmed when the Airbus took off. After touch-down in Brussels, Merkel switched on her iPad; on the way into the city she was following the news.
At 3.30 p.m. Central European Time the news came that Japan had proclaimed a nuclear emergency. In the conference center of the EU government chiefs she followed the TV images from Japan whenever possible. An event had occurred that Merkel had considered unimaginable. She didn't let on and continued the session outwardly unaffected. But by herself, inwardly, she had made a decision.
"That's it!" she said the next morning when she discussed the situation with her staff. In Fukushima, the nuclear age ended for the German chancellor.
Of course, this is partly conjecture: Even Spiegel magazine does not quite know what Merkel thinks when she doesn't let on. But Spiegel certainly has sources within her staff and within the relevant departments who must have told the same story: The decision was made then and there, by Merkel.
Of course, part of the motivation to decide that way was public opinion, and she had to install an ethics committee headed by her longtime party friend Klaus Töpfer that purely coincidentally recommended exactly what she planned to do; she had to build coalitions to break the substantial opposition to the plans and play the usual political games, something she was very good at by then. But it was her decision, she was in the position to implement it, and that is what she did.
That is the reason Germany is exiting nuclear power.
1 According to wikipedia, "one of continental Europe's most influential magazines", "known in German-speaking countries mostly for its investigative journalism". No journalistic lightweight.