Because YC1, YC2, YC3 is a headcanon interpretation of Yonko's subordinates powerlevels, completely ignoring the unique dynamics from each crew.
There's nothing saying "katakuri>king>beckman>Shiryu". Nor anything saying they aren't equals. Each crew have a unique dynamic that reflect on similar total powerlevels.
Kaido's top commanders are only 3. King, Queen and Jack. Big Mom's are 4 Sweet Commanders. Whitebeard has how many division commanders? Blackbeard have 10 titanic captains and so on.
We dont know how the future fights will play out. Will Mihawk maybe join Shanks? in which case, he would be Zoro's matchup. But then, who will fight beckman? Sanji? Who knows.
Sanji fighting King now doesn't mean his next opponent will be weaker just because it might not be a "YC1". Shanks' crew is the most balanced one, so you might assume that his officials are stronger, to compensate for Shanks being "weaker" than Kaido or Big Mom.
The Sweet Commanders have shown some gap in power, but, so far, King and Queen haven't. We don't know if their gap is small like Kaku/Jabura, or bigger like Katakuri/Cracker. All we have is the fact that in a cardgame, the "king" is usually more powerful/valuable than the "queen". But storywise, nothing has been shown to suggest any kind of superiority in power.
Since Zoro is busy top, this is a good opportunity to showcase another SH that could take on such a powerful foe. We know both luffy and Zoro could take on these YC, now we need Sanji to show it. I would expect the other one to be Jinbei, but he is busy with Whoswho. Unless there's a shuffle. The other candidate right now is Yamato. Her first appearance is knocking down Ulti with a single attack. Maybe she is that "another SH" (if she really joins) that can take on these more powerful enemies.
I really wish for Marco to step aside and let the SH's take on these enemies. But we'll see how things unfold.