I'm guessing you've never seen Telegram? It looks like a modern messaging system.

This is a screenshot: https://dribbble.com/shots/12242096-Telegram-Messenger-Free-Download

While Signal looks like any generic SMS app. Heck, both NextSMS and Textra look as good/better and have more "features" (interface customization).

As a technical person who cares more about functionality that stuff doesn't matter much to me, but it's easy to see how a typical user would perceive them (and honestly, the difference is so great it even affects me).

Signal looks dated, like SMS, Telegram looks (and behaves) modern.