In a studio, backgrounds--just like lighting--can be carefully controlled. Outside, however, you have to take the world as you find it: a world that includes cars, telephone wires, and the like--an unending stream of potential image clutter.
So, taking effective portraits out-of-doors means finding good, uncluttered backgrounds (like this one, no? it's a bunch of marsh grass), and employing a few tricks of the trade as well, like using a long telephoto lens and careful placement of the subject to create "bokeh" or a blurred background effect.
It's a challenge, to be sure, but when it works right no sterile studio backdrop can hold a candle to it.