Answer: So that hacks like me--and more importantly, my hacky brethren who endlessly "analyze" the 2008 election on TV--would spread the video posted above.

"Hello Democrats!" Obama said as he strode on stage, amid screams and squeals and a roar of applause. "I just wanted to come out here for a little something to say." Incidentally, that "something" was not his praise for "Joe Biden and Jill Biden and Beau Biden and Mama Biden and the whole Biden family." Nor was it his kind words for wife Michelle, who "kicked it off pretty well"; for Hillary Clinton, who "rocked the house last night," or for her husband Bill, who "reminded us of what it's like when you have a president who actually puts people first." Instead, it was the short statement he delivered last. "We are going to be moving to Mile High Stadium tomorrow, and I want to let you know why," Obama said. "At the start of this campaign, we had a very simple idea, which is: change in America doesn't start from the top down, it starts from the bottom up. That change is brought about because ordinary people do extraordinary things. So we want to open up this convention to make sure that everyone who wants to come can join in the party and join in the effort to take America back."

In other words, Obama was playing pre-emptive defense. With Democrats worried that the move to Mile High contradicts Obama’s convention goal of "connecting with average Americans" and offers Republicans yet another opportunity to characterize the Illinois senator as "a narcissistic celebrity candidate"--after all, the GOP is already calling his be-columned, classical-style stage set the "Temple of Obama"--the campaign clearly wanted a chance to frame the decision to its advantage, and chose the most visible moment (prime time) and most famous surrogate (Obama) to do so. Moving the convention to a 75,000-seat football stadium isn't about showing off my celebrity, Obama said. It's about conveying my message of inclusiveness and grassroots organizing.

Whether voters will agree remains to be seen. It's worth noting, for instance, that Obama's riff about "mak[ing] sure that everyone who wants to come can join the party" was a bit hyperbolic--the event's 60,000 tickets sold out within 24 hours, forcing the campaign to turn away hundreds of thousands of fans. And to viewers watching at home, the difference between a 75,000-person grass-roots organizing event and a 75,000-person rock concert will probably be imperceptible. That said, when the pundits and prognosticators take to the airwaves and the Internets to speculate about the "risks" of Obama being seen with all those voters--as if being popular were a bad thing in an electoral democracy--they'll now have to include Obama's own rationale, conveniently captured on video, in their reports.

Kind of like me.

UPDATE, 1:26 p.m.: Hat tip to reader SMS67: "The stage at Invesco that is being ripped by the GOP" may have been designed "to recreate the Lincoln Memorial where Martin Luther King Jr. gave his famous ['I Have a Dream'] speech on this day 45 years ago." Developing, as they say...