It may have been political. Germans are more likely to trust the BioNTech vaccine than the Moderna vaccine, so taking the Moderna vaccine sends the message that

It may have been political. Germans are more likely to trust the BioNTech vaccine than the Moderna vaccine, so taking the Moderna vaccine sends the message that "All vaccines are good enough", which is the correct message if the intention is to maximise uptake of limited vaccine supplies. This was also probably the reason why she took the AstraZeneca vaccine in the first place.