Because BM is all set up to be given a redemption arc. She was a sweet kid who was horribly misled by Carmel and Strusen, and has a dreadful uncontrollable condition. She has a noble, but twisted dream, because of that. Her big, horrible act of "killing" her son was reversed when we discovered BM can restore the lifespan of people she takes and Moscato was brought back to life- expect to see Opera again at some point. Her crew are, relatively speaking, not that bad- Katakuri respects Luffy, the older ones all seem to be genuinely affectionate towards each other, Lola and her mother clearly got along well, Pudding's been redeemed, Perospero offered to let Brook and Chopper leave WCI even after they fucked up the wedding and tried to assassinate his mother, Pekoms is a decent guy, they didn't actually kill Pound.
So we get a reminder here that Big Mom, without the malign influences of her life, would be a good person. It lets Oda move her about Wano without causing absolute mayhem, before getting her in place for the alliance.