They use the Metal Mario combo simply because it's the fastest. I personally used Luigi in one of my runs to buff up my acceleration a bit to be able to recover from hits. I only found this useful in one case, not that much at that.

lol tfw late replying

a big factor in it unless your playing all cups is RNG and resets based on it, and speed recovery SSMTing with heavy characters and both of the factors in most cases nullifies advantages with luigi over metal or bowser