Storage space can be a real problem, both for new, unbuilt stuff and actually finished models. It would be wonderful if I had enough wall space to display 1/18 scale diecasts. The quality and detail is so much higher the bigger you go, but they do take up a lot more space. Therefore, I choose to display 1/64 diecasts, giving up detail for diversity.
Same goes for model trucks. While I do like variety, I just don't have the room. I display most of my trucks without trailers, because the trailers, especially the long ones, just eat up too much shelf space.