I imagine it would be because the other 4 sacrificed their souls and crystals of Light in order to stop the flood.

It's not explicit, but not only were their crystals faded, they were also damaged, prior to being turned into sineaters. I think it's from the sacrifice, whereas Ardbert's was faded due to being used only for world traversal, I suppose? Perhaps it protected his body from Vauthry's influence.

It might also be because Ardbert had no unfinished purpose or weights that drove him in life. He was pure, so to speak, driven by charity. All of the rest of his companions set out with specific people they hoped to avenge or save. With no driving, unfinished business, his body lay unresponsive to Vauthry's influence.