How did stackoverflow become a success when most of its predecessors/clones are not?

I don't think there is a single aspect which was decisive -- there were several aspects working together to ensure Stack Exchange's success.

1. A large end user need. People are always looking for better answers for common problems, and the programming community was under-served.
2. Smart design, deep understanding of the problem: Watch this Joel Spolsky presentation at Google to understand how much thought went into the design.
3. Celebrity endorsement: Stack Overflow began with the programming community, and Joel Spolsky and Jeff Atwood already had great recognition in that community, due to their excellent past work.
4. User generated content, viral growth: These words are almost cliché in the startup community, but when user generated content can be made to work it works very well indeed.

The magic of usability, responsiveness and other factors. Behind the scenes the surface, it's a simple database application, but on the surface, it's about the ability to make "Friendly-User" software -- NOT "User Friendly". All of us can do "User Friendly", but making a user love an application to use it again and again when they have choice -- that takes special skills.

User rating system is awesome, human nature is to strive for growth and people see it happening over here by getting better privileges as a reward for effort. Once you sign up, it won't be long before you find yourself stuck with this site just because of the nature how everyone gets the reputation they desires / capable of. Negative voting helps to increase the quality of the site which makes it even better.

A piece of software can be as great as it can be but if there is no one to check out then who will use it and talk about it?

Before launching Stackoverflow, Joel had around million pages views per month on his discussion forum and blog. I don't know the exact figure but it was definitely a huge and combined with Jeff's blog traffic it is massive traffic.

So obviously if you show your visitors something new and shiny they are definitely going to check out and if the product is real good then the users are going to be falling in love and spreading it all over the web. That is what happened in my opinion. This just seems all automatic but there is lot of effort spent on marketing and timing the actions.

Every success requires hardwork and lot of effort. And every effort and hardwork needs marketing in one way or the other. To the outsider it always seems like an overnight success but it is not.

Do the copycats have that kind of visitor volume? I guess not