Because Telegram supports many more platforms. I'm using Telegram on Linux (terminal client), Windows, Windows Phone. My wife uses the web client. Other friends use Android and iOS clients.
Signal has limited platform support, e.g. I can't use it on my OS X desktop nor is there a version for the iPad (I could us e the iPhone app but I'm not a savage and the user experience actually matters to me).
Worse though is the complete lack of support for using the same user profile on multiple devices, I want all messages to follow me where ever I go on whatever device.
Signal is nice and all, and I want it to succeed. I try to help in that regard by providing and maintaining their Danish translation (as well as for similar apps and services, except Telegram who despite repeated requests have not opened a Danish translation on Transifex).
iMessage, Facebook Messenger, Google Hangouts, and plain old XMPP all let me switch to my laptop keyboard when my thumbs get tired. That's table stakes in the messaging game. Until the promised browser extension is released, Signal is as worthless to me as Whatsapp.
Really? My experience has been that for some reason, on some phones, messages are only downloaded when the app is opened, which is really annoying but seems to be an interaction-with-the-OS issue rather than a networking issue.
However, messages are always sent and received immediately on phones without this issue (still in my experience).
On my phone (a Nexus 5 running CM), messages arrive promptly on most days, but on some days they only arrive when I open TS. On my wife's phone (different brand) they mostly arrive only she opens TS.
But that's a digression. The point of my comment was that this experience can be rather a turnoff if you happen to notice it really soon after your first install. It's easy to believe that a whole group of people is turned off by one member's negative first experience.
For messaging apps, platform support is crucial.