Another factor that was mentioned when it flew medical supplies to Montreal is that the An-225 always flies with all the crew it needs, including replacements, and so it was feasible for the plane to wait for a few hours while the logistics were being sorted out (e.g. cash-and-carry deals right on the tarmac, the situation was reportedly crazy in march/april). With another plane/airline, the time slot may be too tight because of crew schedules, etc.