Security aside, Telegram is just so much more pleasant to use, and I think that’s what wins users over more effectively than probably anything except network effects.
Usability on Telegram is fantastic in how it syncs messages between devices and allows editing/deleting them. I see its security as good enough for my risk model, which is primarily keeping my chats out of the hands of Facebook, Google, and other predatory tech giants with a history of playing fast and loose with user data.