Why does the Google flight search engine not offer this option to me?
It does offer this and even cheaper one if you search for it correctly
In this case the cheaper options require self connections (i.e. two seperate tickets) in Houston which most search engines don't offer because of the risk involved. KIWI is infamous for self connections.
why is the price difference so big?
Variety of reasons:
- NLU is mostly a domestic airport, not a lot of internationals destinations and airlines.
- "Second airports" are not attractive to international carriers. If they want to increase capacity to Mexico City it's much more efficient for them to add another flight to MEX (2 flights a day or from another hub). Notable exception would be JFK/EWR and LHR/LGW but these are major international airports and hubs.
- You can't leverage a code share since Houston is a United Hub (i.e. Star Alliance heavy).
- Airline prices are unpredictable and illogic in general. The price of the same flight in the same class can easily vary by a factor of 6 based for no apparent or logical rason. It's all based on "dynamic pricing".