Because yelling fire in a crowded theater can cause immediate harm to those around you. Burning a piece of cloth doesn't cause harm. It may make you mad, but it doesn't cause harm. The First Amendment doesn't protect speech or action that does harm. Burning a flag does neither. Really though I don't care what's advocated, I don't fly it in my front yard and I'm sure as heck not going to say a pledge to it. Of course I expect 'conservatives' to try to pass some Amendment on that next.
Burn one in my presence, and it may.
If burning Old Glory is 'speech', than it definitely qualifies as 'fighting words'.