Signal isn't magically protected by proper E2E encryption. When the time comes to take them down legally, all that's needed is evidence from end users phones demonstrating illegal activities occurring over Signal. But - for now - Signal seems to have better friends and or be upsetting less authorities than Telegram.
To me it's pretty clear reason why they've gone after Telegram is the Channels and Groups. Seen from a certain perspective Telegram channels are an alternative to Reddit, and have been popular medium during COVID and the Ukraine war for "alternative news". By now Reddit is properly controlled and subdued but Telegram isn't.