Because Oda hasn't respected Sanji as a combatant and I would like for formidable combatants to actually join the crew.
Quoting myself from the spoiler thread:
The combination of Law, Zoro and Luffy have felt more like a monster trio than Sanji, Zoro and Luffy have for the last
decade. Given Zoro and Luffy's feats now, I don't see End of Wano Sanji being on the same tier with them (let alone their end of Wano versions), so I don't think the dynamic that sometimes existed preskip would be restored.
Like let's look at the best case for Sanji. He leaves Wano at Katakuri level. Well Katakuri level fighters get low diffed by Base Kaido. Both Luffy and Zoro should leave Wano above Base Kaido.
In addition to his Rooftop feats, by the end of Wano Zoro will get:
- Mastery of Haoshoku enhancement
- A black Enma
Luffy was already able to go toe to toe with Hybrid Kaido (apparently without Gear 4th!) for 30 minutes+, and he'll probably eventually defeat him.
At the end of Wano, Luffy and Zoro will be some flavour of Mid Top Tiers.
A YC 1 like Katakuri gets low/no diffed by them. And Katakuri level is the best End of Wano Sanji can hope for.
End of Wano Sanji gets low/no diffed by Luffy and Zoro.
On the other hand, by portrayal, End of Wano Law probably pushes End of Wano Zoro to at least high difficulty, and Yamato has the hype and performance to justify that if Oda wanted to.
Oda could write the story so that Law and Yamato leave Wano as Low Top Tiers (and it's what I'm hoping for).
Given the choice of Sanji being the 3rd strongest Strawhat and being a YC 2 - YC 1 while Luffy and Zoro are solid Top Tiers (he'll be low/no diff material for them), or Yamato and Law joining the crew as Low Top Tiers (so relative to Zoro and Luffy), I'll pick the latter every time.
I do not care for a monster trio where the 3rd strongest is
at best low difficulty fight for the other two.
Tl;Dr: Sanji is too weak to make the Monster Trio feel meaningful.