Well the reason is that there are state laws against outside observers, and no treaty giving any foreign government the ability to monitor. So they’re just enforcing the laws, as they’re supposed to.
Mind you I’m not saying the UN or any other nation is going to interfere, but seems really important to follow laws around voting to make sure the attitude of enforcement isn’t lax.
Well you go ahead and find those treaties for me, since I’ve never gotten a result back from a search. And I’d like to believe but have no proof of that.