McCarran International Airport is the most convenient and the closest major airport to Zion National Park and Springdale. There is a TIME ZONE change when going from Nevada to Utah. Drivetime from Las Vegas to Springdale is 2.5+ hours but you need to plan on 3.5+ as you will lose an hour to the time change.
Salt Lake City International Airport is the next nearest major airport.
Drivetime from SLC to Springdale is 4.5 hours.
The St George Municipal Airport is the nearest airport to Zion National Park and Springdale.
It is a much smaller airport so there are not as many flight options. Drive time to Springdale it 1.5 hours.