You might be interested in Hidden Champions of the 21st Century: Success Strategies of Unknown World Market Leaders by Hermann Simon (link).

Simon defines a hidden champion as having 1) number one, two, or three market position globally, or number one on its own continent, 2) revenue below $4B, and 3) a low level of public awareness. The book is geared towards distilling lessons for managers and investors as you might expect given Simon's background as a consultant and business professor.

The book is mostly focused on how hidden champions are run rather than why they appear in certain geographic regions. But he does take a stab at answering your question of why so many are concentrated in German-speaking nations.

To start with it is not just Germany, but Northern Europe writ broadly. Simon writes:

We find that there is a similar density of hidden champions in Scandinavia, The Benelux countries, and Northern Italy versus those found in the German speaking countries. By contrast, the rest of Europe has far fewer midsize market leaders of this kind. This company segment is little developed particularly in France and the Iberian peninsula.

So it's not so much that only Germany has hidden champions but that France, Spain, Portugal, and Southern Europe are missing them.

In these countries [those missing hidden champions], we observe a preference for and admiration of large corporations and the so-called "national champions”… I consider the role of the state a major distraction to the true course of hidden champions

Simon therefore explains missing champions in the rest of Europe as an outgrowth of industrial policy championing large state-sponsored or semi-public businesses that crowd out mid-size leaders.

What about the U.S., then? Certainly they are free market supporters? Well...

In the US, we find many midsize companies with excellent competencies, who consider their home market so large that they have not seen the necessity for internationalization. This attitude... harbors serious risks. Exclusive presence in the U.S. market makes such companies vulnerable when leading international hidden champions enter this market.

So to sum it up, hidden champions are not just a German-speaking phenomenon but exist in similar concentrations across Northern Europe such as Scandinavia and the Netherlands.

The key seems to be that these nations have the right mix of free-market policy, economic development, and a skilled labor force (I haven't discussed, but Simon also mentions access to talent) to build powerful businesses. And yet, these nations are too small to fully support these businesses with a domestic market, forcing them to compete abroad. This gives those businesses closer ties to their global customers and the ability to see around corners and understand rising international threats. Simon argues that this set of internal and external forces builds mid-sized hidden champions.