There have been a couple of occasions in my research in which I've come across a preprint that is several years old and is very relevant to the work that I'm doing.

Often these preprints have very promising initial results. However, when looking at the CVs or Google Scholar pages of the authors on the preprint, I can't seem to find a version that ended up getting published in a peer-reviewed journal, even if the preprint is several years old already. Why would would a researcher abandon a manuscript that they obviously put a lot of time into?

Do researchers sometimes just abandon lines of inquiry because they get too busy? Or, is this an indication that their promising initial results were not robust enough for peer-review, and I should be wary of attempting a similar study?