Doivid said
I'm gonna take a devil's advocate type switch here and say because that would get tons of public backlash. Also I'm betting it's just not as comfortable for the delegates, and probably provides a little bit of a boost to the host city (not counting all the damage). It would look really bad to have it on a military base when people are already unhappy with the parties involved.
People have notoriously short memories. The minute the news gives them something else to rage over, they've already fundamentally forgotten or stopped caring about their source of outrage.
But really, what's worse? Murmurs of people wondering why they're doing business on a military base and coming up with some stupid theories? Can't be worse than people claiming shit like 9/11 was an inside job, at least nobody dies if they do it in a secure location that people will, once again, have largely stopped caring about in six months.
And like Nex said, there's protestors who show up to those things for the sole reason of causing shit and to paint a bad portrait of the police. Remember that "Pepper Spray" cop that everyone was losing their shit over? If people bothered to look up the whole video, they'd have known that the police gave the protestors who were surrounding and blocking off the police from leaving several warnings to let them out or they'd have to use it. The cop in question even knelt down in front of the protestors who were sitting there to explain to them that if they refused to move, he'd be forced to use pepper spray and what have you. It's hard to claim to be the victim when you are more or less trying to detain the police for arresting protestors and saying shit like "If you let them go, we'll let you leave."
As well, during each of these G20 summits, the cities are trashed and hundreds of thousands of dollars of damage happen, stores are broken into, looters run rampant, and a lot of people get hurt. How is it in any way worth having those stupid summits? Do the leaders actually follow through on commitments, or are they just making appearances? What exactly is accomplished that is worth people losing their lives and getting badly injured, businesses shutting down, and entire sections of the city locked down?
If not military bases, then fine. Pick a more remote town that isn't going to be a hotbed that's easy to secure. Huntsville was a perfect location, since you could only reach it by high way and very few people are going to drive several hours to a small tourist town and risk getting stopped and turned away from a police barricade.
As for the delegates being comfortable, who gives a shit? Throw them in a Super 8 or Holiday Inn. They don't need big fancy accommodations that cost hundreds, if not thousands, a night, on a tax payer's dime. If what they're doing is so important to helping the global economy and stability, then they should suck it up and do their part, too. You can't justify almost the entire thing. It isn't worth it.