Pilots and first officers tend to switch who has flight control on each leg of their flights (which is the Pilot Flying vs Pilot Assisting), and the MCAS system uses the AoA vane associated with the side of the cockpit that currently has flight control.

There is no good reason for only listening to one sensor.

There is a sort of good reason for having a split between pilot/copilot side: the instruments are redundant (both physically and electrically), so in the event of malfunction you can failover to the other side.