Terrance,
I am going to write from personal experience.

Don't worry yourself over the ones who are not interested in attending. It will drain you emotionally if you do. Move on without them.

All families will have people who are not interested in attending the reunions. I think if everyone in the family tree attended the reunions, we all will have gigantic reunions! But, that is not the case. Here are some reasons some folk just don't attend.

1. Many people are introverts who don't mix and mingle well with people, even family members.
2. If it's not a branch of their family they feel particularly close, then they are less likely to attend.
3. Many folk have a "I don't know them people" attitude, and if people in their immediate family don't attend, then they won't attend as well.
4. If they were not particularly close to a parent, then they will probably be less likely to attend a reunion on that parent's side of the family.
5. It is still sad how issues like educational levels still divide families.
6. I think we as genealogists tend to be more excited about reunions versus people who are not. Some folk just do not get excited about family and about "roots."
7. Some family members dislike other family members for various reasons.

Just to name a few...

Thanks,
Mel