because they were state run and had well practised interception systems.

Because the post cooperates with law enforcement. The issue here isnt that criminals used telegram, it's that telegram didnt cooperate with the state. If you want to have a policy of non-cooperation you cant hold the unencrypted data.

I’m sure a post office would definitely refuse a legal request to intercept the mail.

Guess not: https://en.m.wikipedia.org/wiki/Postal_interception