Dragonball has an incredible open tone and is not limited like
One Piece - Pirates
Hitman Reborn - Mafia
Fairy Tail - Wizards
therefore it is accesable to a larger audience.
Dragonball was a cornerstone in manga history - since it established all the now typical things like the naive food addicted main character ect
Also many current series like one piece, naruto ect would have probably never existed without dragonball since their creators were inspired by it
So regardless of if you think that Dragonball is good or bad, overrated or not - fact is that no other series could have played the role it did - becoming a milestone on witch other manga artists could draw on for inspiration and guidence