The Rhine valley is an active tectonic fault, so after Fukushima it does not sound like the best idea to build nuclear power plants there. Furthermore, the region is one of the more densely populated one in Germany and it is one of the economic powerhouses (BASF, SAP) and one of the important transport corridors in Europe (ports of the Netherlands -> Bavaria, Switzerland, Italy).

The Biblis power plant (http://en.wikipedia.org/wiki/Biblis_Nuclear_Power_Plant) is located directly at the core of this region, is one of the oldest reactor in Germany and one of the most powerful. It was built in the early 70s, so there is no protection against plane crashes, no secondary remote control room. If it was destroyed, roughly a million people would have to be evacuated immediately and because it is situated right at the Rhine river, major cities like Cologne and Dusseldorf downriver would have to deal with radioactive water flowing by.

Just a worst-case scenario, but people just felt uncomfortable with risks like that.