Domain names are the real estate of the cyberworld, so it's the same with them as it it with real life real estate; it's all about "location, location, location"..
Short and easy to remember one-word domain names are prime "cyberspace property", and the best ones can get thousands of direct type-ins every day from people looking for the exact item, service etc. as the domain keyword describes (like Diamond.com) so companies and domain investors pays big bucks for them because of that.