The reason for this is somewhat complicated. It has to do with the physics of the lenses and how longer zoom/portrait lenses compress the background and create separation between the subject and the background. Wide angle lenses do the opposite, they pull content in to the photo, thus taking away that visual separation. This is why it is difficult to achieve a nice bokeh in a wide angle shot.
I am experimenting now with a method of taking wide angle shots with bokeh. It is in the experimenting stage and requires a very particular way of shooting with some very particular lenses, and as always some post processing, but I think it may have a place in my landscape portfolio if I can master it.
This is a shot I took last night using this effect. What do you think?