I think we have to remove our nerd goggles for a minute and think about this. We can appreciate the level of effort and detail he put into his costume because we ourselves are fans of the original movie and many of us are kinda used to seeing people in costume, given our interests and the common way we express those interests. But most people are not nerds. And here's a few reasons why non-nerds, who probably run this theater and would actually be the majority of moviegoers, would not want TRON guy to come in costume:
1) The lights. They would be distracting.
2) It's a grown man... in costume. I'm sorry, but to a non-nerd, there's still something more than a bit creepy with seeing a costumed man in public who is not part of an organized publicity push for the movie. Especially for parents, it makes things more than a little unsettling.
3) He would likely be the only one in costume. If this were an organized costume event, or a movie where many fans are likely to show up in costume anyway, they would probably look the other way. When I went to the first showings of the fourth Harry Potter movie and the third Pirates of the Caribbean, half the crowd was in costume (and this was also in Minnesota, my home state, btw). But this costume is more-or-less one of a kind. TRON is more difficult to make a costume for, and he's one of very few who have been able to pull it off. It would be distracting for that reason alone.
4) Honestly, most non-nerds have likely not heard of TRON guy, or it was so long ago they just forgot about him. He was kinda big news to us nerds, but not to the average Joe. The most they probably heard about him was a little blip on the local news, and he was forgotten by the end of the month. So even if they saw him, they probably would only barely recognize him, if that.
That's just the way it goes. You can say it sucks, but that's life.