Because very few people ordinarily use encryption on a daily basis (and it some countries it is illegal), so using encryption in a visible way is itself a beaco

Because very few people ordinarily use encryption on a daily basis (and it some countries it is illegal), so using encryption in a visible way is itself a beacon that attracts suspicion. The suspicion is likely to attract more undesired attention from authorities in this situation. That alone could be fatal to the goal or the person even if the encryption is never broken. So better to hide completely the existence of the message. This is a method of exfiltrating messages without being noticed.