1. The invite was not personal. Consider reaching out personally to some key people and ask them to invite others personally. That direct touch still matters, even in this electronic age.
2. You didn't answer the question: What's in it for me? You have to demonstrate or explain why there's value for them in attending.
3. You failed to focus enough on who else will be there. People want to know if key peers will attend as well. They want to be able to connect, communicate, and learn from them. Yes, we all still act like high school students. We want to know who else is going before we join the party.