Let's remember these are human beings. Yeah we love 'em, but going to a giant fan gathering might not be up his alley. Especially being up there in years.Frank Welker? He's actually really embraced the convention scene in recent years. He's planned for other cons in Knoxville, Boston, and Toronto later this summer.
It really can be something as simple as him having a personal obligation, a possible work obligation, or like others have said, a health issue. I don't think anyone needs to read into it. Guests cancel on conventions literally all the time for a multitude of reasons.