Why did my IAH-SYD flight avoid Mexican airspace?
Though routing this far north seems to make the flight about 200 miles longer, flight duration doesn't exactly correlate to route length. For a flight this long, wind plays a large role.

Take a look at the image below of the current wind forecast from ADDS, with the two flight paths in blue. You can see if they took the southern route over Mexico they would be flying directly into a pretty strong headwind. But by going further north, they can fly in much lighter wind conditions.