Honestly I think this is because Sanjis character is designed to be someone who breaks mental limits to grow stronger while Zoros character breaks physical limits.

Sanjis body and fire will grow stronger naturally as his emotions break through the mental blocks that limit him.

Zoros body and haki will grow stronger as he finds his limits and in dying moments learns to shatter them.

Meanwhile Luffy is both which is why kaido specified that the mind and body must both be strong for devil fruit awakening.

That’s just a personal take though
 
Honestly I think this is because Sanjis character is designed to be someone who breaks mental limits to grow stronger while Zoros character breaks physical limits.

Sanjis body and fire will grow stronger naturally as his emotions break through the mental blocks that limit him.

Zoros body and haki will grow stronger as he finds his limits and in dying moments learns to shatter them.

Meanwhile Luffy is both which is why kaido specified that the mind and body must both be strong for devil fruit awakening.

That’s just a personal take though
:crazwhat:
 
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