Because oda say wano will be biggest arc in OP , he bring everyone to Wano. Messed up everything. Actually, when Jimbei talk BM to leave crew, after WCI , just i think Luffy will go Wano . At the same , BM punish Fishman Island to rebel. And Luffy move BM after Kaido.
Now , everybody come Wano. This arc does not finish for a long time.d