The NRA's type of organization means that they are legally required to hold an annual meeting for members. It was far too late to change it to somewhere else, and it was against the law to postpone it. They cancelled every single part of the meeting in Colorado which they were able to and significantly reduced the length of the function.
Also, it's false to equate violence with the tools used to commit violence.
Edit: Of course that's not to disparage Manson in any way, he's entirely free of blame too, I just don't like misinformation existing.