I don't know what kind of usage numbers their secret chats have, but if that's the case maybe that's a sign most people prefer the convenience of non E2E chats to the security of E2E? And if someone is just a tiny bit interested and motivated the secret chats are there if they want them.

In fact, I'd argue that it's better to use encrypted chats in a common chat client than having to explain why you use a specific encrypted client if you ever find yourself in a situation where that's necessary, like a border crossing or police interaction.