On Telegram, login on a new device and you can see your all messages automatically. On Signal, you seems to need to perform it manually, and your history will lost forever if you phone are lost or broken.
I think the first kind of "backup" is what most people would expect, because it is convenient. But to provide this convenience, it can't be full E2E, so Telegram supports secret chat when you need max security if you are willing to sacrifice convenience.
You can get the same convenience with the Signal/Whatsapp E2E type of app. All you need additionally is to keep a passphrase stored somewhere. As long as you do that you can have automatic backups that allow you to setup a new device without access to the old one and get all your history back, without losing the E2E encryption benefits by having unencrypted backups on a server somewhere. Signal UX for this is poor though.
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.