From what I've read, WhatsApp won precisely because of being truly platform independent. 10y+ ago Android/iOS monopoly was not yet that huge, and compared to other apps, WhatsApp had a client (which worked well) on the long tail of cheaper devices/OSes, which drove its adoption, because you could communicate with literally anyone.
And the final, genius and/or lucky move (in Brazil) was when phone operators started allowing both Waze and Whatsapp [1] to work even when your data plan had ended. Now everyone, even those on the cheapest pre-paid plans, was guaranteed to get your message.
[1] Maybe some federal judge considered those apps essential because someone was unable to text for help due to their data plan, and operators were ordered to allow that traffic to go at all times.
Telegram is faster and adding features all the time - people only it during Whatsapp outages - returning as soon as the service is back.
What Telegram does much better are public groups for creators, but its never the primary channel to communicate with their audiences, since discoverability is much worse than Instagram/Twitter.
