He didn't have an offer from Wun Wun to fight one on one for Winterfell. If he'd shot Jon Snow then the dozens of Archers surrounding him at that moment would've just killed him. By challenging Snow, theoretically, if he wins he gets to stay alive.
Realistically, the archers would've unloaded on him the moment he shot the giant and not give him an opportunity to challenge Jon, but that wouldn't have been as fun, would it?
Realistically, the archers would've unloaded on him the moment he shot the giant and not give him an opportunity to challenge Jon, but that wouldn't have been as fun, would it?