Preference, storage and money. This is not to say there aren't other sized posters that I like, just not enough to delve into them. Also, when I think of movie posters, I think of the posters in the marquee frames outside theaters when I was a kid as well as now. I don't think of gigantic 28 fold posters from England when I think of movie posters. I'm currently hovering around 200 one sheets including video posters, and I'm probably going to sell a few just to make sure I stay under 200 for some time. 200 is my nice little "limit" and just so happens to be the amount of posters I've found worth buying.