Using Telegram is a rational decision if you want a service that's good at fun conversations. Signal's value proposition is _secure conversations_ and it does that much better than other services. "Fun" is not part of Signal's value proposition. More people want/need fun conversations than need secure ones. Regardless of what people "should" want, Telegram serves people's mundane everyday needs materially better than Signal does. "It's fun and the people I care about talking to already use it" is a compelling value proposition, not a frivolous one, given people's everyday needs.

The more someone cares about security and is willing to trade away other good things for security, the better a platform Signal is — but remember, this also flows the other way.