We have too many holes We can't give away draft picks to trade up. Besides, it remains to be seen whether Bush has the build to be an every down back in the NFL.
The Saints are aware that it's theoretically possible that the Texans won't take Bush or trade down (ie say they take Williams first overall). In that case, they'd get Bush without having to give up any picks. They aren't trading out of that number 2 spot until they are sure Bush won't fall to them. The only way we get Bush is if we trade with the Texans.