Great question! Essentially the Solus team has adopted the policy of not giving ETA's for a few key reasons. First and foremost they are volunteers and do this during their free time. As such, sometimes life inevitably comes first and it would be a disservice to give a deadline and then not be able to meet it. Some distros go by release cycles every "x" amount of times per year to put out updates and new features. However the problem with that is, if a feature is not ready yet for stable use, it has to be pushed back then to a later release. Telling the community something will be released at a certain time, but then failing to meet that deadline honestly puts unnecessary pressure on the lovely volunteers that help to run Solus. By adopting the "when it's ready" approach, it allows the devs the time they need to get their tasks done without having a deadline looming over their heads.
I may have miss one or two reasons, so my apologizes for that, however that is my understanding of the matter and I hope that helps clear anything up, and hope y'all have a great weekend!