Three reasons:
1) Texas has more resources to deal with a natural disaster than Louisiana
2) We have more guns
3) With Katrina, a lot of people (especially the wealthy) evacuated which left behind a certain demographic that has a history of committing the majority of violent crimes in the US. When that demographic was left to rule over themselves for a week, New Orleans descended into third world anarchy. That didn't happen in Houston due to 1) and 2)
However, looting has still been common in certain areas of town, especially my area since we're still dealing with reservoir runoff