Yes, they want to let the public in, and yes, they want to showcase the enthusiasm for Obama. But I have to think they also want a very white crowd. Not only did they hold the speech in a stadium, they reserved most tickets for locals in what is a very white city.
This isn't a criticism. Probably the biggest impediment white voters have to voting for Obama is the primal fear that he's "for the blacks," not for them. (See this study of voters in Macomb County, Michigan.) Surrounding him with white supporters is a pretty good way to counteract that fear.
Update: Anita Dunn from the Obama campaign writes, "Denver is not a white city. Ask former Mayor Webb or Pena. Close to majority minority. We took it to Mile High because we wanted to open up the convention and party to more people."
I checked and, yes, they're right and I'm wrong. Sorry for the sloppy assumption.