The US Title is a championship that is exclusive to the Smackdown brand. It appears in Smackdown Vs Raw 2006, Smackdown Vs Raw 2007, Smackdown Vs Raw 2008. It is the mid-card title for Smackdown. Anyone can compete for the title.
The US Title is a mid-card title, and anyone on Smackdown can compete for it. Since Smackdown has a decent sized roster, the competition for the title is ok.