I don't think anyone cares about E2E encryption as much as tech people think.

For all of the much-vaunted complains about their lack of it, I am not aware of any proof or credible claims that Telegram the company has ever revealed the contents of non-encrypted messages or group chats.

Meanwhile, I don't think any authorities actually care about to what extent E2E Encryption makes it harder for Signal the corporation to extract message data. There's plenty of other ways to skin that cat - on-device compromises, abuse of backup mechanisms, abuse of mechanisms to manage linked devices, etc. They'd go after them just the same if they thought there was anything they really wanted on there.

If there's any real difference, I think it's most likely because many more of the group chats that such authorities are aware of and find "interesting" are on Telegram because basically nobody really does E2E well in medium-large groups right now.