The original dual lands are very powerful in the game and haven't been reprinted since Revised so there aren't a lot of them. And people pay that much for them.

The original dual lands are very powerful in the game and haven't been reprinted since Revised so there aren't a lot of them. And people pay that much for them. Modern dual lands come with restrictions/penalties for taking advantage of their dualness (time delays, damage, choosing which version to use when played, etc) and they are often some of the most valuable cards in a set.