Well, it turned out I was wrong. They nailed it with an IR air-to-air missile. I tip my hat. I suspect the balloon was reflecting the IR from the sun, which would have stood out against the cold sky background (very cold at high altitude).

This means they could have done it before it crossed into U.S. airspace. So, they allowed it to progress along its path to see what path it would take. There is some suggestion that they may have been collecting communication intelligence from it (e.g., capturing its uplink signal). Not a bad approach: let it do its thing, find out what that is, and pop it at the end of the show.