More or less for the same reasons that Trump slammed the Turkish economy (with tariffs): disapproval of the current Turkish approach to the rule of law, in particular as it applies to citizens of concern (Germans to Germany):

Merkel said on Aug. 29 [2017] that there would not be any progress with Turkey even in trade relations, if the rule of law was not guaranteed in the country. Good ties are “linked to respect for the rule of law and this is not guaranteed right now in Turkey,” she said.

Germany has issued new travel advice for its citizens and is blocking talks on expanding the EU Customs Union with Turkey. “I do not see a mandate to expand the Customs Union in the current circumstances,” Merkel said.

Merkel also reiterated Germany’s demand for Turkey to free journalists and rights workers from custody, including Die Welt daily correspondent Deniz Yücel.

“Our demand is very clear: That the people who have been arrested there are released,” she said. Yücel, a German-Turkish dual national, was arrested on Feb. 27 on charges of “making propaganda in support of a terrorist organization” and “inciting the public to violence,” after initially being detained on Feb. 14.

Yücel, who faces up to 10.5 years in jail if convicted, denies the charges.

The German government has demanded Yücel’s swift release, as well as the release of other German citizens detained in Turkey.