Because japanese voice actors > english voice actors.
Not always and the fact that you said it so boldly makes me believe you rarely watch anime in dub and are just basing this off of a few previous experiences and/or people's opinion.
DBZ- Much better Dub
Cowboy Beebop- better dub
Baccano!- Sub is worthless, only way to watch this is dub
Kenichi- better dub
Granted that's not a high percentage when compared to all anime in existence but those are just the ones that stick out the most for me, I'm sure I could find plenty more if I took the time. That said I do think the majority of anime sound better as sub but not all japanese voice actors>English voice actors.
OT: Watch them both, Usopp has Krillin's voice in the Dub and Zoro is Piccolo so it's already off to an ok start seeing as they're established voice actors and come from one of the most recognized anime dub projects. It depends, when I want to watch it watch it I watch sub, when I want to watch it in the background and do some other things I put it on Dub so I still get the gist of what's happening.