German users - why did the FDP collapse in 2013?

[Beezer]

I'd say everything's been said already. Ultimately the party doesn't have much of a core electorate (unlike the Greens of Left Party) so they depend on syphoning off votes from the CDU in particular to cross the 5%-threshold. Quite a few center-right voters who felt the CDU had become too Social Democratic jumped on the Liberal bandwagon in 09. When the FDP turned out to not deliver on any of its promises, their voters just asked "what's the point?" and rejoined the CDU, stayed home or defected into the AfD camp.

[Beezer]

The FDP did quite well in the east for a while if I remember correctly (there were two "liberal" bloc parties in the GDR after all with plenty of members and a decent sized bank account). So I'm sure that the average eastern FDP voter isn't exactly a Westerwelle small government liberal but a former LDPD-NPDP member, hence these folks don't have much of a problem voting for the Left Party which is in some parts of the east first and foremost seen as an East German rather than a Socialist party. I doubt too many Wessi-FDPler on the other hand would join the Left Party.