Nobody said they weren't selling well.
And as for "doing something right", that would be marketing. Whenever I go into shops, Markers are the most prominent. They are the most flashy/noticeable looking binding compared to the no-frills, but practical, appearance of other bindings that perform the same or better. They also have a lot of technical jargon that sounds nice. This may just be me, but it appears that store clerks always try to sell Marker bindings first, and then recommend something else if you don't bite on it. Same thing online, a lot of ski/binding combos are solely for Marker.
All of this would lead to sales, but not necessarily a good binding.