I think the reason can be found in the deepest corners of the male stereotype.
Shonen are manga that are meant for men/guys.
Men associate themselves with strength, coolness and toughness, which are main character traits of the male stereotype.
Zoro is the embodiment of an ideal stereotype. Add to that badassness and a cool character design. In Dragonball his equivalent is Piccolo, they are very similar and both have green appearances, furthermore they both protect small mascots (Gohan& Chopper) and have a closer relationship with them.
Chopper and Gohan are the embodiment of Shonen Manga readers, anything but male. Piccolo and Zoro are the pure male embodiment in the Shonen manga they read. Unattainable male stereotypes.
That's what makes them so exciting and fascinating for Shonen readers, who are mostly boys.
Shonen are manga that are meant for men/guys.
Men associate themselves with strength, coolness and toughness, which are main character traits of the male stereotype.
Zoro is the embodiment of an ideal stereotype. Add to that badassness and a cool character design. In Dragonball his equivalent is Piccolo, they are very similar and both have green appearances, furthermore they both protect small mascots (Gohan& Chopper) and have a closer relationship with them.
Chopper and Gohan are the embodiment of Shonen Manga readers, anything but male. Piccolo and Zoro are the pure male embodiment in the Shonen manga they read. Unattainable male stereotypes.
That's what makes them so exciting and fascinating for Shonen readers, who are mostly boys.