Because the G20 is not made of allies and it sends a bellicose message.
How would you feel if our President had to go spend several days on a Chinese military base? How bout if we were on bad terms with the Chinese?
Also, what does that say to the world when we're so scared of terrorist, we have to hide our leaders in secret bunkers? Wanna talk about looking wimpy?
NYC is fully capable of security in this case.
Well honestly I'd feel safer with him on a military base than in...say...downtown Beijing. But I see your point.