It's not as if the office is all powerful or anything. Or as if Hesse had a tradition of direct mayoral elections that predated the tradition of low turnout in lesser elections - things may sometimes be different in rural settings, but in the cities and the suburbs mayoral elections have the lowest turnout of all.
(And I think that if the CDU had gone with someone less prominent and thus divisive, treasurer Uwe Becker, say, they would have won this election. On an even lower turnout.)
It's funny how it's basically the reverse in France. Turnout in 2008 mayorals was around 65%, for 2010 regionals it was below 50%.
France is one of the few European countries which still gets over 50% turnout in local elections afaik.
