Basketball is huge in Spain and Italy, how big is it in Japan ?
Anyway I think it's more about gaming culture and trends.
It's known Spain is very much into internet gaming for several reasons, it doesn't come as a surprise it is the largest community here.
Outside of internet infrastructures or lifestyles, Europe has a developed market for management games of the BB's type lead by the famous Football Manager serie.
The USA, for exemple, has a long history of fantasy team games which were almost non-existent in Europe until recently and that style of games is much more popular than FM clones.
I'd suspect Japan is closer to the USA in term of gaming trends and is more familiar with fantasy teams and as seelenjaeger said Japan simply plays other types of games as part of its gaming culture.
There are many things to consider, the UK have a large internet gaming community but basketball is non-existent in the UK and they are small on BB despite being the birthplace of the game genre.
Poland is another singular exemple, basketball is only moderately big there (our Polish members will contest this ;p) but very much into internet, I remember a study showing that Poland had the highest number of searches by searchers in Europe ahead of the UK which came as a surprise to me. Poland has a fast growing internet market and maybe BB is a witness of that as I don't remember Poland being amongst the largest communities only a couple of years ago.
Philippines users number stands out on BB probably for the fast growing market too.
Lithuania has a small population, only a few millions, but basketball being by far the major sport, it is reflected on BB. Greece and Turkey are two other exemples showing that the popularity of basketball in a country will draw users towards BB.
Germany, on the other hand, where interest in basketball is mild has a disappointing community size.
So... many many things to consider...