Why has Germany not complied?
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Whistleblowing is reporting on misconduct. Generally it takes place within an employment setting. As long as employers enjoy the assumption of truth and protection from transparency and accountability, undermining the law with lies is too easy. There is a lot of resistance to transparency and accountabiltiy. That having been said, not implementing the directive does not release the countries from their obligation to enforce it.
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German labor law currently gives employers sweeping rights to terminate employment for alleged disloyalty.
Any criticism of an employer is grounds for immediate termination. Pointing out that an employer is slandering you will get you fired for disloyalty.
If you provide evidence, that the employer is violating your rights, the employer can fire you for revealing "company secrets." It is a Catch 22.
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You do not have to do anything wrong. All your employer has to do is allege you did. The labor courts will most likely uphold the termination.
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Implementing the Whistleblowing directive will de facto change established German labor law and practice, but it will take time to change it.

The Whistleblower Directive includes a reversal of the burden of proof for legal proceedings. The Directive prohibits employers from taking or even threatening reprisals, such as dismissal, against whistleblowers. However, if the employment relationship is terminated after a report and the whistleblower claims that the termination was a consequence of his or her report, the Directive introduces a presumption that the employment action was in fact an (illegal) reprisal for reporting or disclosing breaches. It is then up to the employer to prove otherwise and rebut the legal presumption.

Although a directive doe not have legal status in its own right, if a member state does not implement the directive in due time or lawfully, the legal situation is different. If the deadline for implementation has expired, the European Court of Justice assumes the directive has direct effect if its provisions are drafted in a clear and unambiguous way so that they do not longer require further specification or clarification from national legislators.