New aircraft have different configuration
As I understand it, Only the latest delivered (or soon-to-be delivered) A380's with the higher weights and PIP can make it on the long IAH flight, and these aircraft are configured slightly differently than the older ones due to relocation of some IFE equipment they can fit an additional seat-pair somewhere in the back. The older ones with the fewer seats have the range for other markets, such as SFO, but not quite for IAH.
As an aside, judging my personal experiences traveling on the Dugong to L.A. it seems to be carrying full loads on that route with no load restrictions, so I suppose they want the new planes for the Houston flight whereby they don't have to fly around with empty seats.