There were multiple foreseeable issues which German Rail (DB) failed to foresee and which together made it uneconomic:

• The service would have crossed the Schengen border, requiring DB to arrange for border controls and customs at significant cost (including infrastructure at stations).
  
• The trains earmarked for the service (Siemens Velaro D) should have been equipped with extra safety equipment at significant cost (also in extra weight).
  
• Commissioning of the Velaros for Belgium and France (for existing services currently operated with older ICE3 trains) was problematic enough, also at significant cost.
  
• DB doesn't have all that many high-speed trains and realised it needs more for domestic service.

As for the new Eurostar trains, they aren't suited for the electrification system in Germany.

_{*Lunatic*, n.
One whose delusions are out of fashion.}