Signal is not as convenient. People want convenience.

I really tried to like Signal and convince my family to use it, but the lack of iOS backups, CarPlay support, and web client were dealbreakers for me. I'm no Meta fan but I've found WhatsApp to be the best mix of secure default settings plus usability. It's ugly as sin and doesn't have a Linux app, though.

Signal is better than Telegram in terms of security and privacy.

But people use Telegram for its plethora of features, the UX, and its attention to even the smallest of details.