First of all, we know that human levitation is possible and known. In The Prizoner of Azkaban, we see Sirius Black use Mobilicorpus on Snape's unconcious body to levitate it away from the Shrieking Shack, bumping his head intentionally along the way. It is safe to assume that between Dumbledore and McGonagall, one of them should have known the spell, though we (the audience) had never seen it at that point in the series.
Getting back into the tunnel was difficult. Lupin, Pettigrew and Ron
had to turn sideways to manage it; Lupin still had Pettigrew covered
with his wand. Harry could see them edging awkwardly along the tunnel
in single file. Crookshanks was still in the lead. Harry went right
after Sirius, who was still making Snape drift along ahead of them; he
kept bumping his lolling head on the low ceiling. Harry had the
impression Sirius was making no effort to prevent this.
Harry Potter and the Prisoner of Azkaban
Second, the source and nature of Colin's curse was unknown, and so they would have likely sought to err on the side of caution. Any magic performed on him could have caused further damage to him, or to the caster. Since the Hospital Wing was so close, they would have wanted to get Colin into Madame Pomfrey's expert hands as soon as they could without exacerbating the situation.
My third thought is a bit more speculative, but keeping their wands free (not being used on a continuous spell) means that they would have been able to defend themselves against whatever attacked Colin. This argument is a bit more difficult to defend as their hands would have been occupied, but wand in hand while carrying the boy would have still afforded them the ability to cast a shield charm. Casting Mobilicorpus could have made their reactions a tad slower.
Lastly, in situations of high stress, many wizards resort to muggle practices. Harry Potter and George Weasley get in a fist fight (muggle dueling) with Draco Malfoy after a quidditch match when Draco insulted both of their families. It is possible that the situation with Colin Creevey was dire enough that thought of spell casting was forgotten in the need to rush him to Hospital Wing.