Heh. In the DC world, Superman is kind-of-a-big-deal. In the MARVEL Universe? Just another alien clown with a cape that can be taken down by some of the weaker characters. Thanos would consider Kal-el a speed bump unworthy of notice.
Dag-nabbit! I HATE letting my inner geek out before Halloween!
Because when Thanos was first introduced, the creator had made him a lot smaller than he is now. He was told that if he wanted Thanos to be a more intimidating character to beef him up a bit and do something like Darkseid.