Few reasons might be because:
- I think they just shortened the name from SonGoku to Goku.
- Other reason that might be worth noticing that his real name is Son Goku. But Krillin, Yamcha, Tien, Bulma, Oolong, Chaotzu and Master Roshi are the only ones which call Son Goku. Other one's (example: villains) call him simply Goku.