How many times has this been asked on this forum?
Simple answer, is because wing down is easier to maintain stability right down to touching the ground.
Alternative is having to use a boot full of rudder at the last moment, then sudden use of aileron to stop the wind blowing you side ways when you straighten up.
Larger aircraft have more momentum, meaning they are less prone to being pushed sideways in the wind once you straighten. Also some large aircraft (737?) have twisting main wheels, so you don't have to straighten till you touch the ground.