To answer this question, let's take a look at two images of the model Andrelica:
Andrelica looking sultry while posing with a Mexican stool.
Andrelica looking just plain amazing.
The answer is, I think, twofold. First, most of the time people are standing or sitting in a more or less upright position. So naturally, there will be more vertical photographs than horizontal photographs. Second, most Web sites constrict the image size based on width. So, if a horizontal and vertical image are given the same width, the vertical image will be larger overall, and therefore have more impact. Photographers know this, and will usually give preference to posting vertical images over horizontal images for this reason.