The data indicates it has a good following in SA, a decent audience in some of the EU, and a cult in Japan.
I'm sure it's a nice series but it doesn't seem to be much of an anime, then again if we are looking at places like SA, who had very few shows back in the day, i can see why this blew up since they didn't have much in the way of cartoons or anime back then, so something like this would have stood out.
It blew up in every single place where soccer is popular.
It's basically a shounen like DB except with sports.
You have goalkeepers training by blocking rocks, breaking their hands and their shoulders during a game, people stepping on them and almost bleeding them to death with spikes, guys using their legs like a bow to shoot dragons on goal, guys who kick the ball with a spin like a bullet to hurt whoever is in the way, and then stuff like
It's no different than Dragon Ball really, they start "sort of" normal and they end up throwing fireballs at each other all the time.
American football had something like this, Eyeshield 21, practically every single sport had something in the same vein. Golf, table tennis, boxing, judo, swimming, rugby, volleyball etc.
There's more realistic anime takes on the sport, but people love Tsubasa because it's so bullshit.