In Germany it is mostly referred to as the BioNTech vaccine. I think it's more about international publications being more likely to use the name they recognize or think their audience will recognize.
Most everyone is going to know Pfizer and Oxford respectively. Internationally Mainz isn't exactly a famous city, nor is AstraZeneca a well-known pharmaceutical company.
> Internationally Mainz isn't exactly a famous city
It might not be famous but it played a key role in the development of the Renaissance, Reformation, Age of Enlightenment, and Scientific Revolution, as well as laying the material basis for the modern knowledge-based economy and the spread of learning to the masses.
AstraZeneca is as well known as Pfizer, they are big pharma after all, the subset of people who know about Pfizer and not AstraZeneca is going to be quite small.
Most everyone is going to know Pfizer and Oxford respectively. Internationally Mainz isn't exactly a famous city, nor is AstraZeneca a well-known pharmaceutical company.