I wonder if fewer people who want to fly international trips (than people who want to fly domestic trips) think of looking up flights at Southwest.com.

Remember, Southwest availability doesn't show up on most aggregating websites (at best, some like Kayak may present an ad to also check Southwest), and so anyone in the habit of simply checking aggregator websites (Kayak, Expedia, Travelocity, Orbitz, etc) may not be aware that Southwest is now an option for that international flight.

Southwest service in the US existed long before internet booking, and so people were well aware of it. But Southwest international service started long after internet booking, but people traveling internationally were already set in their ways about where and how to look for flights, so they're probably not looking specifically at Southwest and therefore not seeing Southwest availablity at all.

IMHO if Southwest wants to get serious about international, they should rethink having their flights available on Southwest.com only. Even if that approach works for domestic, it may not work for international.

Southwest's "domestic" footprint is pretty well defined in people's minds, since Southwest covers so much of the uSA. Which "international" destinations they fly to from where is hazy in people's minds, so why would the average "international" traveler want to go to a separate site for each trip only to find out that only to a fraction of those destinations does Southwest offer service?)