Why don't Cleo and Rikki tell Bella about Emma?

Cleo and Rikki don't tell Bella about Emma initially because they want to protect her from the complexities and potential dangers of being a mermaid. They fear that revealing Emma's existence and the challenges they face as mermaids could overwhelm Bella or put her in harm's way. Additionally, they may be concerned about how Bella would react to such a significant secret, complicating their newfound friendship. Ultimately, they aim to keep Bella safe while navigating their own unique circumstances.