.....
All of the designers have a silent weep for a moment.
....
Haha... just kidding.

But for real, this is one of those things we designers discuss often when we scoop together and make card packs.
I know this SEEMS like it would be easy to do, but it's a serious mental block when designing cards (for me personally) sometimes because not all designs 'flip' nicely and fit both ways. It really limits what you can put on cards and can be a lot of work trying to tweak all of the cards to fit both ways. I know some designers include all cards in both orientations, and those designers ROCK (cough, cough, Traci Reed!!! :wub

but for some it just doesn't seem like packs sell any better when we do. I think it would almost be easier to include both horizontal and vertical cards (not with the same designs on them) but it seems most people prefer vertical cards. It's definitely something we talk about behind the scenes!
I love knowing what customers think about it.