Apparently the Pentagon wasn't able to put 2 and 2 together on this, and it took intervention from South Korea, who pointed out this was a really bad time for such an exercise, to convince President Trump to cancel it.
The Trump-Kim summit is in just three weeks, and North Korea was already questioning whether the US was in the right frame of mind to talk after the last war games. South Korea said the exercise would generate too much tension ahead of the talks.
US officials say they accepted South Korea's call because they want to keep up a cooperative relationship with South Korea during the diplomatic process. They added that the B-52 bombers had a routine training mission near Okinawa in recent days instead.