AR used a small portion of their phase one drilling to fully evaluate Mexus' huge property and then left because they saw no potential or..
Yup, that's what I'd go with.
Many, many exploration targets are abandoned after the first phase of drilling. Throwing good money after bad is not good business.
From the one and only map that Mexus ever released, it looks like Argo drilled across every major mapped geological structure. They were testing both for potential high grade along structures, but also looking for disseminated mineralization. Based on the drill data that was released (if you ignore PT's fraudulent "averaging"), it does not appear that there is a disseminated target, and the high(er) grade was limited to one drill hole.
So.....Argo left. Maybe their other business issues made the decision easier.