Because more voters had made their crosses on Schroeder's part of the ballot

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"Perhaps" it also played a role that Kohl was chancellor for 16 years already and the economic disadvantages of the reunification of Germany became more and more apparent. Schroeder appeared more dynamic.
If you want to hear anything about the donation scandals of the CDU (Kohl's party) I have to disappoint you. They had no influence on the election as they were discovered later.