Why were most of the characters' names changed in the dubs?
Captain Tsubasa IS a very popular anime that was said to inspire a sense of sportsmanship in children. Anime is well known for its influence on the audience.

When it was dubbed, (in Hindi (Indian)) the names were not changed since they could very easily be distinguished in my language as something foreign. The pronunciation varied drastically though.
I suppose the names were changed to make things easier as it was aimed at a younger audience who might have trouble remembering names they don't often hear in their regions. 
If I recall clearly, there weren't any changes in the English dub either. 

It also depends on the dub team. If I were young and I saw complicated names in a cartoon (not familiar with anime then) and others, I might as well prefer something else without giving it much time or chance.