Why is Graz’s historic center “World Heritage”?
Well, here is the justification…

The historic centre of the city of Graz reflects artistic and architectural movements originating from the Germanic region, the Balkans, and the Mediterranean, for which it served as a crossroads for centuries. The greatest architects and artists of these different regions expressed themselves forcefully here and thus created brilliant syntheses.

Here’s the link. Actually a pretty interesting site… I recommend to surf along and read the different jury votes for sites all over the world.
And don’t forget… there are many endangered monuments on our planet. Scott’s hut in Antarctica, for example… but so far it’s not that cold in Graz. (No Al Gore jokes, please!)