Nothing prevents Telegram from not implementing that architecture and reading every single group message and non-secret chat. Telegram is sketchy in many ways, I wouldn't trust that easily.
Because the founder is Russian and Russophobia is very strong in the USA and Western Europe.
People find other reasons for this (“they made their own crypto”, “marketing says secure but it isn’t by default”) but the core of the argument is that Pavel Durov is Russian and he’s not trusted. If it had been Elon Musk who had created it- I have little doubt that it would be the hottest thing on the market.