It is the simple fact that old coins were often made from precious metals, such as gold or silver. Old coins can often be worth more melted down than they would if they were spent like regular change. However, the fact that they are still stamped coins makes them even more valuable. And their value is only enhanced even more by the fact that they have been around for a while.
If the old coin is rare its value is much higher. The fact that coins are old tends to make them rarer. The older a coin is, the more likely that people have exchanged it for more modern currency and the more likely that the government has gotten a hold of it and melted it down. In fact, most governments have specific legal requirements to destroy old coins in order to keep the money supply modern, making it more difficult for coin collectors to find old coins.
