I’m maybe not the best person to answer but I think the general idea is that it’s supposed to be an open communication for any licensed person to use and any that anyone is allowed to listen to, and it’s supposed to be used exclusively for non-commercial purposes, and there’s some additional regulations about who can use it and how

If you’re sending encrypted transmissions, no one can tell if you’re using it appropriately or not. You may be transmitting on frequencies your license doesn’t give you permissions to use, you may be using it for commercial purposes, you may not be identifying properly, etc.

Radio frequencies are a somewhat limited resource. There’s only so many frequencies allocated to amateur use or to different commercial uses, etc. If someone’s using amateur frequencies in a way they shouldn’t, they’re tying up those parts of the spectrum so that properly licensed users can’t use them for the purposes they’re reserved for

And if they’re encrypted you can’t really differentiate the lawful from the unlawful users making it harder to police.