Tennis is not very popular anywhere. If we use the number of top players each region has as proxy for how popular tennis is relatively, then it's not doing that bad in eastern Europe.
At least in Western Germany, there has been a tennis boom in the 1980s and 90s when Boris Becker and Steffi Graf were active and successful. Thus, you can still see the inner German border between GDR and FRG on the map.
E.g. England that invented it having a really high density or Eastern Germany being basically blank (main tennis hype has been in the 80's in Western Germany).
This map would actually be interesting if it were normalized against population density. Tennis court density per 1000 inhabitants. Probably need to do it per 100.
Go see in person or find better data. Spain is a desert between big towns. Poland has only 35ish M people on all that land. You can drive for hours in west Balkans without seeing a single settlement. Croatia, for example has less than 4M people in an area which is bigger than the Netherlands. The Netherlands has like 18M people.
You mean like official EU data? https://ec.europa.eu/eurostat/databrowser/view/demo_r_d3dens/default/map?lang=en And "go see in person" is a very bad advice to anything data-related in most cases. Compating population density anywhere in Europe with Netherlands isn't fair.
Poland, Hungary and Romania (and north Balkan as well, it seems) have denser population than rural France, for example. Spain is less densely populated, but still has about as many tennis courts, so it must have much more per capita. It just isn't a population density map. It is another Iron curtain division map, but even so, Czechia and Slovakia stand out as exceptions. There is interesting information in there.