They ran out of samples
Their test is destructive, they can't test the same sample twice. They planned out a number of samples to test for each time period and tested them. At the end of six weeks, they were done.
It doesn't really matter
The point of the paper is that HCV is infectious long-term on surfaces when not cleaned properly, indicating there is a risk of fomite transmission long after an infected patient leaves the area.
They also show that if surfaces are cleaned, the virus is killed effectively. The authors are not advocating to quarantine a hospital room until any residual HCV dies from air exposure, they are advocating being sure to appropriately clean hospital surfaces to prevent potential disease.
It isn't typical (in fact, it is impossible) for a single scientific study to answer all open questions in one shot. If someone wants to put an upper bound on the duration that HCV remains infectious in the world they can do a new study with longer time points.