Gort
those can usually be grabbed at a 30 dollar discount or more if you look in the right places
Sadly, that's probably the reason right there. They don't want to offer a more affordable option.
The question really is: would they make more by offering omnibus (omnibuses? omnibi? omnibussy?) alongside or instead of their current trades?
They must've done the math and figured it wouldn't make them more money. But one could probably make the argument that having an affordable option would bring more readers in and eventually lead to more sales.