My only question is: why Telegram? Why did it become the de facto main platform for ICOs? Why not discord for example?
I would argue that both Telegram and Discord are measures of a successful crypto ecosystem. Investors use both to research a platform. talk with the developers and decide whether to invest their hard earned capital. Discord is starting to displace Slack since Slack is known to have security vulnerabilities.
I agree. I personally prefer Discord over Telegram (because of how simple it is, doesn't need phone number, more privacy, user interface, etc).
Also generally, the more platforms - the better right? Just have everything just in case, why not? Maybe except slack though like you mentioned.

for me personally, phone number is one of important thing to prevent fake account/fake person.