The primary reason for this is that we have a hard time justifying spending the time needed to take the module to the next level when there's no direct financial reward. We have spent hundreds of hours improving and supporting the module. But, for the last several years, we've had plenty of paid client work to keep us more than busy, and thus, we've had very little free time to improve and release the next version.
Buy selling the module we expect to be able to dedicate some serious resources to the Articles module and many other modules that we just haven't had the time to put the finishing touches on in order to get them ready for release.
In the last couple weeks, we've been knocking out all major bugs, making several key improvements to the module, and adding to new Article's module content to the website. Some of these changes, like the forums, are already available, others will be rolled out very soon.
We greatly appreciate the support and feedback everyone has given over the years. We hope that you'll agree that the improvements we'll be able to make to the module through the revenue generated will offset the fact that it will no longer be free.