Probably cause thematically we seem to be heading back to Wano more or less:
- Orochi is alive still
- Momo is about to save Wano
- Hiyori, Onimaru, Tengu, etc will probably play their roles by the end of the arc.
- Scabbard roles will conclude, whatever it is
- Fire Festival and its meaning
Also just the fact that it's possible some of these fights eventually drag out into Wano potentially. Who knows, maybe Kaido orders all 10 Numbers to trash the capital if Onigashima can't, prompting the scabbards to protect it or something. Maybe the BMP are also there too.
I get what you are saying but I think this is why Oda did the act separation. I still think it's "possible" Act 3 ends when Onigashima crashes or is about to. Who knows