Imho part of the reason for that might be because Telegram has gotten the reputation of being the "encrypted Terrorist IM app" [0]
Especially in the mainstream media, I've seen countless news pieces which blame Telegrams encryption for making it supposedly impossible to track down terrorists [1].
In contrast to that Signal and WhatsApp don't nearly get mentioned as often, at least in regards to "encryption enables terrorists!" FUD.
That's an important point. I think the thing about Telegram is that it would indeed much less willingly cooperate with governments than say Whatsapp or FB Messenger do, since they don't have much relation with the western governments after all. However that says nothing about whether it is theoretically secure to hacking, which is quite a different thing.
In contrast to that Signal and WhatsApp don't nearly get mentioned as often, at least in regards to "encryption enables terrorists!" FUD.
[0] https://www.hsdl.org/c/extremism-and-encryption-terrorists-o...
[1] http://www.smh.com.au/technology/technology-news/telegram-th...