Oda has been constantly giving Sanji some of the biggest fights, and in the Wano Country arc, he reaffirmed his position as one of the Wings of the Pirate King, meaning that along with Zoro ...
Essentially, Sanji would be fighting this battle not just to make sure that Luffy becomes a pirate king, but also to save Pudding. Since Kuzan was the one who abducted her, it makes a lot of sense.