Because the original Dragon Ball struck the perfect balance of adventure, action and comedy, couple that with the fact that it came out in most regions outside Japan when Anime was gaining traction and was so different from American animation that was in a phase of removing violent content (remember how Spiderman TAS wasn't allowed to show people actually punching eachother?) and it instantly became a hit with the audience.
Now, by the time Z rolled out people were so invested into these characters that they stuck with the series, which traded comedy and adventure in exchange for taking the fights to the next level and it basically was "Hype; the show".
Additionally, Dragon Ball is unlike other shows, including Anime, in which the characters have aged with the audience, Goku started as a kid when we were kids, now goku is a grown ass man with children and grand children, people relate to seeing the characters grow up with them, shit, Mexico even makes commercials aimed at parents who are showing the show to their kids.