1. Passenger jets often take a bit of cargo, but even if they rejiggered, it'd only be half full of cargo.
2. Many companies already hedged the price of fuel so the oil glut isn't really helping them any.
3. Demand is unidirectional: medical supplies are made in a handful of countries (ie China) but there isn't a huge demand for air freight the other direction. See also the container problem.
4. The airline network is a complex emergent system. Passengers from one international flight often connect into another domestic one. Cancelling an international flight has unpredictable knock-on effects for downstream flights.
5. Cancelling a flight can lead to losing your 'slot'. Similar to how google will kill non performing ads, airports have a system to allocate arrivals / departures to airlines that factors in usage. If you shift a passenger jet flying EU to US into doing cargo hauls, that slot is jeopardized.