They didn't work out near as well. Most web users know the com. and feel comfortable with it and they think the others lead off where they might not want to go.
(this is not a scientific reply, just my observation)...KF
When I bought a .com from somebody who was a bit faster than me - okay, a few years ahead of me - I paid an non disclosed, but nice amount for it - and then I registered the .net, org, etc version of it since none of them were taken. Pretty funny. Made me laugh. But i'm crazy, so never mind that.