It's so simple. Writing, publishing and selling books is all about location, location, location.
Think about the last time you were traveling by bus, train or airplane. You wander into a tiny airport bookshop or newsagent's shop, and pick some reading material from the rack/shelf -- quickly. Lots of people do.
Or think about the last time you "wandered" into Amazon.com and got lost among the infinite shelves. It's a different shopping experience entirely.
At the airport, you have limited choice and time, so you MUST decide fast or spend the journey with nothing to read.
If online bookstores are like vast caverns you can go spelunking in for days, then airport bookshops are more like disarming a ticking bomb in a moving elevator: "Pick the blue book! No, the red book! Hurry! We're almost there!"
So from a writer or publisher's standpoint, you want to get into those "bottleneck" locations where the potential reader won't have too much choice or time to think.
Problem is, the well-known commuter bottlenecks are expensive locations and pretty much occupied already (by Stephen King, Danielle Steel and the usual suspects).
There must be some other, unexploited "bottlenecks" where potential readers are simultaneously bored, stressed and frustrated, and are about to enter a period of forced passivity -- similar to when they are waiting to catch a plane or a train ride... and would buy a book if they had a limited selection.
I can't come up with one, but maybe you can...