It's a drive of over 15 hours (one-way) from Washington Utah to Portland. They'd have had to pay for gas, food, and probably an overnight stay in a hotel.
The shortest flight I saw from Saint George was almost 4 hours ($222 round trip, so $666 for the three who would require tickets). There are no direct flights. Plus there still would have been food costs for the layover in Salt Lake City.
Seeing how those kids seem to scream constantly in the car, and Asuelu spends his whole drive time swearing under his breath, the flight was the best option.