My guess it's because the production cost is too high, and counterfeit euro coins aren't a big enough problem to warrant them. Also having a germany-only euro coin in wide circulation would be a bit backwards and impractical, so they would have to implement them in all 19 euro countries.

To demonstrate feasibility and estimate costs. The tech could then eventually be rolled out but since Germany is part of the EU for EU-wide legal tender all mints need to switch: "standard" euro coins have a common design and reverse, and a national obverse (with some restrictions).

As a case study before introducing it into general circulation.

Developing a new coin for hundreds of millions of people in daily use is a big task, it’s easier to start with a subset of 80 million potential users to demonstrate feasibility, and test.

Who says they don't have plans to apply this technology in quantity production in future (just not yet)? I consider it as a very sensible choice to use a one-off collectors item to make a first test towards mass production.