[aptwebapps]

Of course a piece of software that you intend to distribute to others must be reusable - that's the whole point. However, there are plenty of software systems that are one-offs. In the case of Stack Overflow, they wanted to see if the overall solution (which is much more than just the underlying code) would be good for different types of users or whether they had built a one-off.

With Discourse they are setting out from the beginning to make a reusable piece of software but that doesn't mean they are guaranteed to do so. Hence the extended development time before they offer it more widely.