TBH, I was a bit surprised to see the strong reception of the Epic relative to the pretty weak introduction of the Captivate/Vibrant.
ATT/TMO had no problems keeping them in stock, even at ridiculously low prices. I think a large part of it is Sprints yearly upgrade program, so there are a lot of Pre owners who are becoming eligible right now. Moreover, a lot of people bought the Pre because of the keyboard, so that might explain why so many are excited about the Epic. On the other hand, ATT has never had a viable android device before the Captivate, and The Vibrant is far and away the best thing on TMO. You would think that with Sprint having the Evo there would be relatively little excitement for the release of a largely similar phone.
I think you said it best with the Pre comparison.
The Pre was a huge hit for Sprint but a year later there is still no successor, so many owners (myself included) are looking for the next best thing and thanks to yearly upgrades why not use it on this.
About the Vibrant and Captivate not being as porpular I think it is due to the carrier. I am not 100% sure but as far as I know Sprint is the only major cell phone carrier that allows yearly upgrades unless you go with a one year contract on another carrier.
Also add in the fact that unlike the other Galaxy S phones this has a keyboard which I think has a huge effect. Besides the Droid there are not that many high end Android devices with a keyboard.
When you add in all these equations I think the Epic is just a good decision by Sprint to release it when Evo is having inventory issues, there are very few high end keyboard Android phones, and many Pre owners from last year are looking for an upgrade.