There were a number of things that led to Telegram's (relative) success. One of them is the owner itself - Whatsapp has suffered in reputation with the sell-out to Facebook, especially when they announced to combine the data stores of both companies. From then on, it was widely seen as "yet another thing of Facebook where they want all your data" despite Whatsapp actually offering better encryption than Telegram. Telegram in contrast is seen as "independent" from any major political power - Durov's fight with the Russian government is well known, as is its opposition to Western demands.

Another issue was cross-device compatibility. Whatsapp used to be completely immovable between Android and iOS, and its web UI for a long time was tethered to the phone, meaning it would drain your battery. That has only changed recently, but (IMO) too late.

And the final thing were the mass-fanout capabilities. Telegram groups allowed quick dissemination and discussion of information, for open groups even without requiring a Telegram account. Whatsapp had neither. Then add to that the resistance of Telegram towards any kind of moderation, especially from Western governments... and suddenly a bunch of conspiracy theorists, vaccine deniers and outright Nazis had the perfect platform. No moderation from the platform side, no content requirements other than "please don't spread CSAM", and effective management tools for group admins to yeet off counterspeech attempts. These days, you have QAnon groups numbering six-digit subscribers [1], and the same for German copycats [2].

Telegram's success is mostly based on outright ignorance and defiance of the rules everyone else (including Facebook) plays by.

[1]: https://www.logically.ai/articles/new-telegram-research-show...

[2]: https://www.tagesschau.de/investigativ/kontraste/querdenken-...