I already got to the bottom of it.
1. They had no idea that the etiquette would be to invite you. They probably actually didn’t invite anyone and just let it get around by word of mouth. Since you aren’t a particularly close friend, and are not part of their group of friends, you didn’t find out.
2. They’re embarrassed by your email, since they now think they did something wrong. And are not sure how to reply, hence the no reply.
3. Why did you email them? Ok, you missed the party. What do you want them to do or say?