For magazines that are both printed and distributed online it makes sense to just throw the files for the printed (portrait) version online rather than do all new layouts in landscape with the incumbent effort and expense.
If you have a screen with sufficient resolution, it's easily possible to just scale the magazine and you read it in half the width of the screen instead of using the full width, whilst getting whole length visible. I guess you could also just rotate the screen (some will without too much hassle) to portrait.
For online-only publications one does wonder why people haven't started innovating with landscape formats. Probably because the software packages are set up for printed publications (i.e. portrait) and its easier to simply go over from the legacy formats for interoperability. If there comes a time when printed is pretty much dead and people use exclusively electronic means to read magazines or books, then landscape formats might come along. Of course with the rise of e-readers and tablets, it's no hassle to go from landscape to portrait (standing a laptop on end is less ideal), so the portrait formats may well live on.
"A fear of weapons is a sign of retarded sexual and emotional maturity." Sigmund Freud
Shooting is my meditation