Why did the fire only damage the upper levels of the coliseum

Severe damage was inflicted on the Colosseum by the great earthquake in 1349, causing the outer south side, lying on a less stable alluvial terrain, to collapse. Much of the tumbled stone was reused to build palaces, churches, hospitals and other buildings elsewhere in Rome.

Easy cases are almost always resolved correctly by lower courts. If there doesn't appear to be a possibility of the lower court having made an error, the Supreme Court will decline to hear the case. Also, the Supreme Court has far too little time to hear every case sent to it, so it will tend to turn down cases that don't deal with important and difficult Constitutional issues.