For me, there’s two big reasons for this:

Signal chats are E2E at all times, while Telegram is only E2E when you explicitly create a “secret chat” with whoever you’re conversing with. I don’t fault Telegram too much for this, because they still provide the option to use E2E for everything, but Signal gets brownie points in my book because they just do it by default without getting in the way of the User.

Secondly, as far as I know, Telegram uses their own in house encryption techniques as opposed to industry standards. I am not at all knowledgeable about encryption or cryptography— I only know what’s required of me in my job (basically the bare minimum), and so I don’t actually know whether this is anything of serious concern. It could very well be that Telegram’s encryption techniques are just as effective as the established norms, but I do see the general consensus trending towards “roll your own encryption = bad, use established norms = good”, which is primarily what I am basing my opinion on here.

To further detract from my own point, it actually seems like Telegram might be using “established norms” for encryption nowadays anyways [1], although I couldn’t really tell from the brief description I read on Wikipedia.

Overall, I think Telegram is perceived as being less secure than Signal primarily because of the reputation Telegram has for implementing their own in house encryption techniques, even if they don’t use those techniques anymore— their name has become associated with their known history of using ad hoc encryption.

[1]: https://en.m.wikipedia.org/wiki/Telegram_(software)#Architec...

Telegram has e2e encrypted chats but only on mobile and not on desktop for some reason.