The huge difference between mail or phone and telegram is that both mail and phone work with law enforcement, with useful records being made available upon subpoena. Telegram, by design, will not.

If you think drawing that parallel is useful to Telegram, they would then also be required to maintain the same standards of security as the mail, with package inspections, drug dogs, entire teams of government officials investigating illegal activities etc.

The criminals use it precisely because it is not a parallel to other available channels, as it circumvents those safeguards.