What's important here are two things: your willingness/availability to attend the reunion, and the willingness of others to accept your non-attendance.
You can do precious little about the second, and have already made your decision regarding the first. Now the challenge is to convey the first item in a way that helps with the second.
I believe that the reason you are unavailable is irrelevant and talking about it only encourages argument. So it's time to quit saying, "I can't make it because I have..." and time to start saying, "I'm sorry, but I'm not available and won't be there." "Why not?" "It doesn't matter; I won't be there."
That's it. You're not available. The reason is irrelevant - you can't come.
Now, for the people who won't accept your lack of availability or are overly insistent on your attending, I'd say something like, "Thanks for asking. I understand it's important to you but I can't make it. Please don't keep asking; that won't change that I cannot make it." And if people try to "help" you by saying "change your plans" or "come on, don't you want to...", I'd respond a little more forcefully: "I've already said I won't be there and I'm going to ask you to stop and to accept that I will not be there. This is getting tiresome. Thanks." Keep in mind that the badgering will continue until people discover one of two things: you will be there, or it won't work. So it's important for you to help them arrive at one of those two and quickly.