I second this: it's because C is a simple language. Modern languages are a cluster-f*k of features and there are still opportunities to code in security holes. Frankly, if you were to boil down the "unsafe" C vulnerabilities it is a small class of exploits, suitable for automated detection.

The interesting thing about C is that it is one of the few languages which has been able to survive without massive changes throughout the years. All these hyped-up modern languages are in flux ALL THE TIME.