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.

They generally don't have to proactively police it, but they have to answer court orders in every country that has courts, or they'll be in trouble in that country. And countries are free to cooperate with each other to enforce these.

Pavel Durov was arrested when he traveled to France because Telegram was noncompliant with French court orders. You can ignore them in Russia... you can't ignore them in France. And you can ignore Russian court orders in France but not in Russia. And the Russian or Indian court is free to ask the Montenegrin government to suspend your domain name and the Montenegrin government is free to agree or disagree.

Signal is already well known to governments. In fact a few years ago there was a report in the UK media about how some governments used signal instead of official channels like email and did so because of Signals disappearing messages feature (ie making those MPs less accountable).

in fact, telegram does support e2e encryption ("secret chats")