Because Smoker is much stronger than Zoro
Does Zoro have any bone in his body that is anywhere near as hard and tough as Sanji's shin you think? Sanji is doing a couple thousands of tons lifts with his one leg, is just a kicker, just hardens his legs and shins and minimaxes on lower body strength
Then Vergo comes and breaks him, as in not even Tashigi had any bones broken or nothing when he wife slaps her with haki, but Sanji who is minimaxed on lower body strength does a perfect block and Vergo performs a dismal looking attack where Sanji has the priority and strength in the block, Vergo has little to no hardness and power in the sort of kick he delivers. Sanji still breaks
So Tashigi comes out like some sort of durabiltiy freak compared to Sanji who has his absolute strongest body part completely torn to shit by Vergo using a weak clash with his ankle. Sanji's HARDEST and STRONGEST part breaks
And Tashigi is a crybaby, Smoker's girlfriend, a his ball sack holder
Just think about that