It took the german goverment years to force Telegram to comply with their requests to get data and shut down specific groups. Also based on your location things that might be legal are not legal in a different location, from a German legal perspective there still lots of things happening in Telegram that are atleast a grey area(piracy, hate speech, porn) or straight up illegal(drugs, terrorist groups propaganda).

I am not in favor of the arrest or any goverment repression btw, I am just adding why goverments in europe might want to do stuff like this.