I would say #1 is fuel, #2 fuel, #3 fuel, and #4 fuel. The higher you are the less fuel you burn. Some/most routes could not be flow if the plane was forced to stay too low. As the plane burns more fuel they will request higher throughout the flight and try to stay right in the maximum efficiency range for fuel burn. Because of this, dispatchers/airlines try to keep as little fuel on board as possible to allow them to fly the highest.
100% Agreed.
The higher the cruise, the lower the burn, and the faster the aircraft can travel.
Take the B747-200 for example:
Below FL100 = 250 KIAS
Above FL100 and Below FL180 = 330 KIAS
Above FL180 = 0.70 Mach (336 KIAS)
Above FL240 = 0.77 Mach (330 KIAS)
Above FL280 = 0.82 Mach (326 KIAS)
Above FL330 = 0.85 Mach (305 KIAS)
(For those who may not be aware, "FL 100" is not 100 feet, add two more zeros to the "FL" number.
FL100 = 10,000 Ft, FL330 = 33,000 Ft.)
Typical cruise for the B747-200 series is FL350, or 35,000 Ft. At that altitude, 0.85 Mach = roughly 555 mph.
Mach 0.89 (587 mph) is reachable, but not as efficient. The airplane is certified for flight as high as FL450.
Weather would be the number two reason. Generally, the air is smoother up high.